Years ago, I went to Oklahoma City and attended a couple War Bonnet tech sessions sponsored by the PCA. There I spoke with such luminaries as John Paterek, The Andersons, David Seeland and Allen Caldwell.
A fellow named Al Zim had with him a German dude named Von Nolting who rustores 356's in Stuttgart. He also had a video tape of the process. I picked his brain.
Ok, first remove the seats and carpet and rear pads. Remove the rocker panels. Survey the damage to the longitudinal outers and the corners of the floorpan inside.
HOW? Lay a droplight under the car and look for pinholes from inside the car. You may have to chip out ALL the bituminous sound-proofing coating the inner floor and any undercoat on the underside although the factory did not always apply undercoat to the bottom.
(this will keep you busy a couple days heh, heh) use a '" wood chisel, cup knot brush and a scotchbrite wheel. Survey the damage, carefully examine the seam around the underside of the car where the floorpan attaches. Look for areas that will need TLC before you can reattach a new pan. By TLC I mean patching seam strip or longitudinal/seam flange.
Ok, this is where it gets dicey. You gotta decide if ripping out the entire pan is worth it, or can you replace 1/2, 1/4, or just small segments. You can buy a full pan, or a 1/2 pan (left or right side). (Pelican, Cy's, Stoddard or your favorite)
Usually, floorpans are bad in the extreme corners, most notably in the front where the wheelwell and forward firewall attach. In this case, you could be in for MORE than just a floorpan. I re-fabricated from flat sheet the curves and flanges that needed TLC. . .trust me, this is tedious work, but cheaper and quicker than doing a whole floorpan, wheelwell AND longitudinal. Speaking of Longitudinals, these little devils like to rust out at the front end where they attach to the wheelwell, the telltale sign is a small hole visible just aft of the front tire.
When you have decided where to patch and where not to...
Cut the pan or section out. (this little sentence makes a mountain of work)
HOW? (use either or in combination)
Method 1) Use a 1 to 3mm thick cutting wheel on a mandrel inserted into an angle die grinder, using 20,000 rpm to surgically remove metal in parallel with the seam flange, leaving the flange as a place to weld in your new pan. Later, you can chisel out the remains of the flange, but this depends on your eye's detection of rust damage. You may be able to just flush grind away the remains of the pan flange leaving the 'good' metal.
You can also run into an unfortunate situation where there is so much rust cancer that there is almost nothing to work with when welding in the new. This is where your guts are gonna tighten, if you dont get all the cancer, shame on you, your welds wont hold. . .and you've created a freeway time-bomb. In this case, just keep on cutting until you get all the really bad metal cut out and patch it with new steel.
Method 2) Use a spot-weld cutter and drillcut every single spot weld on the pan flanges being careful not to disturb the base metal. . .dont cut too deep. -this could be tens to hundreds of welds depending on the mood of the guys when they spotted the car together decades ago. Use the chisel to split any remaining recalcitrant seams.
When you get the segment/pan out, examine your new pan for how it fits. Thoroughly clean the sites where welding is to occur by using any method that works. .scotchbrite, sanding disk, sandblast, spit and/or prayer. . .you can resegment the new pan to patch the holes in your car, or if you fabricate small sections to fit, replace only those bits. This requires a bit of skill.
This is where your draftsman skills come to fore. .use the machinist scale (metric or english) to measure and sketch the hole you just made. There are a variety of clever methods to depict on paper the shape and dimensions of the hole you just hacked into your car. Fabricating these is yet another art. Depending on how complicated the target patch is, will determine how hard and how much time it is going to require. Some forethought before you cut out the old stuff is a good thing to do. . .cut only what must be removed. If you dont destroy the bad chunk, you can use it as a pattern to forge a new part. Did I say keep the old parts to model new parts from?
The important thing is to know every single place where the new stuff fits up to the old and the welds are going to be to tack it back in. Then get out a drill with a nice sharp bit, preferably a stepped Uni-Bit, because now you are going to create the 'spot' welds without a spot-welder.
If you are flange welding, pick your points to spot weld (use the old spotwelds) and center punch/drill a 1/4" (7mm) hole at each site. These will later become 'rosette' welds, you will MIG through these holes to the new metal, and then fill the hole and voids. If you have the skill, the finished weld will be only slightly above the original panel surface and require very little follow-up grinding.
If you are just patching a segment, you will probably do mostly 'butt' welds. To be effective at this and avoid warpage in the panels, you must space the patch edges no more or less than 1/16" (2mm) all around the periphery where the butt welds will occur. Start by placing a 'tack' weld at 2 inch intervals, alternating around the patch like you were torquing head bolts. This will minimize warpage. Follow up by MIG'ing in between the tack points, but dwell only a short time to avoid overheating the panel and warping it or blowing a hole in it. Use a wet rag to cool the welds as soon as you de-energize. -did I say keep everything as clean and cool as possible?
Ok, time for a little admonishment. . .what you need here to be effective is PENETRATION, by that I mean fuse the new metal to the old. To do that you must get things molten hot, not hot enough to blow a hole*1, just hot enough to flow everything together real nice. . .this is where that wide-screen, gold-plated, low-shade-factor welding hood viewplate comes in handy. With it you can see the temperature of your weld by how red/orange it is, you can *see* that steel just starting to boil -oops! too late.
*1) blew a hole eh? use a thick copper 'spoon' and hold it over the hole from the backside. Take the mig stinger and zap around the periphery of the hole, dont worry about penetration yet, but try to fill in the void by tickling the trigger and feeding some stock into the edges of the hole (its ok to use a thin weld rod too). When you get the hole mostly filled, lay into the hole a lot of heat and pile on the slag. If you have to, stop and grind off the gross excess and start again until you narrow the hole down to size, keep trying.
There is no substitute for experience/practice here. A major problem with swapping floorpans is that they are on the underside of your car. Unless you have a rotisserie (like me) you are gonna have to occasionaly weld upside down (inverted) while on your back. You better have your ducks in order, like leather apron, gloves, and crotch armor to keep from tattooing yourself permanently.
As an addition to my 'floorpan swap' tech article, I neglected to mention you should treat the finished structure with a primer of choice, but since this is just a floorpan, you dont necessarily need to paint it with factory colors. You do want to coat it with something to prevent rust. Originally, the pan has just primer and undercoat then color (in that order) on the underside of the car. Inside the car, it is much the same with one major exception: soundproofing.
The factory applied a thick bituminous soundproofing sheet to the interior surface of the pan. This material is available and I highly recommend it. 'Wurth' sells it in packets of six self adhesive sheets. . .horrendously expensive. A domestic alternative is marketed by 'Evercoat' which can be had at your local friendly local automotive paint supplier at a significantly lower cost, but still expensive. It comes in two thicknesses. I use the thicker stuff, because the cockpit of a 914 is noisy enough; the thicker the sheet, the greater the noise attenuation.
The Evercoat material comes in roughly 12 inch sq. sheets, which means you will have to tailor and tile it in. This is a job best done in the sun on a warm day. . .maybe a HOT day so the stuff gets soft and pliable, after all it is made of TAR (thus the 'bituminous' term) and guess what tar does when its warm? The 914 floorpan has a lot of contours to follow, and that means pushing it in with your fingers or thumbs, or using a selection of brayers to roll it down. A heat gun can substitute for a warm and sunny day, but you get into problems with evenly warming a full sheet of this stuff. When cold, this material can break.
Anyway, each sheet has a peel off backing and the trick is to position it ONCE, then stretch it into the contours while it is warm. If you tear it, it can be patched.
Get it too hot and it melts. Try to work with it too cold and it breaks. You cut this stuff with a stout scissors, or a knife (an X-Acto works great) and it can be softened enough that small chunks can be layed in and press the seams together to make it look like one continuous sheet.
This material is also sold by high-end stereo outlets like Crutchfield, known by some trade name, but it is basically the same sound deadener sheeting.
This covering could also be used as a substitute in place of the rubberized sheeting on the rear firewall interior, but the caveat here is that in event of extreme operating temperatures, the adhesive may fail, or the bituminous could melt and sag. It will probably burn if given opportunity.
Once you have this material in place, you can proceed to paint it like any other surface, but the coatings may need to set a bit longer while the volatile solvents evaporate.
Pelican Technical Article:
914 Floorpan Replacement
Porsche 914 (1970-76)