Many of the same 'prerequisites' mentioned in the
914 Floorpan Replacement Articlestill apply -with one major addition to the routine.
To replace an inner or outer longitudinal, you first need to stabilize the body, so the doors still fit when you get done. In the Factory manual section 8, there is a reference to a 'door frame tool' of sorts (I dont recall the exact nomenclature) This tool bolts to the door hinge mounts and attaches to the lock post. I highly recommend using something equivalent. I suppose you could tack weld the doors shut and later cut the tacks. It depends on your level of fortitude.
Replacing an outer long is simpler than replacing an inner. Mostly because the floorpan is attached to the inner, and you have to wrangle with that as you do the swap. I opted to patch only selected bad sections and spots on my inners since the floorpan was in fair condition. (my '73 car)
The 914 longitudinals are like a pair of long clamshells with a spotwelded flange along the upper and lower edges. These form the 'frame' of the car between the front and rear wheels. Removing the entire longitudinal will include the jack mount. These too can be replaced 'while in the neighborhood'. The front end of the long may also have a solid plate spotwelded over the end as an added brace. These tend to rust away because of their proximity to the forward wheel well. These are fairly easy to fabricate from sheet stock. (20-21 Ga. steel, 1-1.5mm) Along the upper flange is also mounted the threshold plate and below that a few braces, possibly with holes to accomidate the air conditioning hoses. Inside the long is a heat conducting hose and a support that is riveted between the inner and outer. A replacement long will not come with the braces or threshold plate. . .which means you will have to fabricate or transplant these items. The threshold plate is brazed at the forward and rearward ends. Braze contaminates steel to the point it is nearly impossible to MIG to, so it either has to be throughly cleaned or re-brazed during the reconstruct phase.
Begin the longitudinal swap process by first removing the threshold plate using the aforementioned rotary spotweld cutter. These cutters have a spring loaded guide pin that you should center in a center-punched dimple you put into the center of the spot weld you wish to remove. The cutter is less than $20 and chucks into a drill. The cutter eats the steel away from the periphery of the weld. Once again, dont cut too deeply or you will have a 3/8" hole thru both layers of steel. These cutters are rather delicate because the teeth are similar in style and pitch to a woodcutting saw blade, tempered and brittle. You must try to get all the teeth to seat on the target surface at once or they break off and then you need a replacement. Keep replacement cutters on hand, because you will need them. The key to using these is to watch the teeth depth, or watch for a puff of rust as the cutter dips thru the first layer of steel. There may be many or few spot welds to cut in this manner, and then you must cut the threshold plate at the forward and rearward ends, nearest the joint seams. I suppose you could heat the braze and separate them, but I had success with just using the 1mm cutting wheel on an air angle die grinder to do the job. Then you must also cut the spotwelds along the braces just below the plate, you have the option of cutting them at the longitudinal, or cut them from the threshold plate. I prefer the longitudinal since I'm assuming you are doing a full longitudinal and this will re-use the positioning of these braces. Once all the weld sites are clear you can take a pry instrument (I had a favorite 1/2" wood chisel) and 'pop' each spot site until somewhat loose. Each of those spot welds you cut will now look like a hole or a steel tablet, depending on your skill.
Next you are going to cut the longitudinal's spot welds along the flanges. This should leave you with the front and rear ends of the longi to vertical cut. Remember to cut the two rivets holding the internal warm air tube's support to separate the two longi halves.
Depending on the severity of rust damage, you can do a partial or full swap. I prefer to replace only that which is horribly involved with rust. This leaves undisturbed the parts of the car that are factory constructed. . .you can't go wrong unless your seam welds are really poor. By this I mean that to cut a section out of the car and seam (not flange) weld two panels edge to edge is a delicate situation because the car is depending on these joints for structural integrity and that is difficult to achieve in a butt weld unless you are very good at fusing the two ends into a single panel with original strength. In cases such as these, it is advisable to overlap then rosette and lap weld the adjoining sections to guarantee structural integrity. By the way, if you cant get enough heat out of the MIG into a weld site to ensure penetration, it is ok to oxyacetylene weld. The 356 rustorer I spoke with used MIG and oxy welding almost interchangeably, but I think oxy overheats the panel and alters its strength character (although Von Nolting believes opposite).
Now that you have the longitudinal (or section) off you must prepare the car for the new part or section. It is advisable to take account of the condition of the inside of the car's clamshell. This is your golden opportunity to peer where no man/woman should ever see, but all 914'ers wish they could before they buy the car. . You can also treat these surfaces to a rust kill coating of your choice, but I dont trust a slathering without a prior cleaning to bare metal. This will allow you to know for sure if there is any bad rust of the structures, and if they need to be replaced. Sandblasting works great, but has its tradeoffs, scotchbrite wheels are fine provided you can get it in there.
The goal here is to get everything as clean as possible for the rustproofing and welding process. A weld-thru coating, zinc paint or POR-15, etc is fine, but clean that stuff away from actual weld sites because they will contaminate a MIG weld. Furthermore, these coatings can burn from the heat of welding and can cause a 'flash' fire, or enough smoke to contaminate your MIG weld or worse, set fire to the warm air tube within the longi and stink up your car.
On the car, every place where a you cut a spotweld must now be transcribed and drilled into the new longi's flange to act as pseudo-spot welds (rosettes). This could be upward of 40 spots, or holes. Or you can skip drilling and use a luggable spotwelder, but these are notoriously bulky due to the transformer needed to generate the weld current. Either way, you need to grind the leftover spotweld pellets down flush with the panel or flange so the surfaces can fit flat against each other. Once all this arduous task is complete, you will need to clamp the clamshells together to check fitment. Dont forget the warm air hose bracket.
If all is well, then you are ready to tack weld the clamshell back together. Use a wet rag to cool the weld sites and minimize burning and warpage. Keep a fire extinguisher handy too.
Alternate the tack welds as if you were torquing the bolts on a head. When the longi is re-attached and all spot welds or holes are rosette'd it is time to grind the welds back to flush with the flange so the threshold plate can be reattached. Use a similar method to re-attach the threshold plate, depending on how well things fit back together, you can start by brazing the ends back on, or spot-rosette the plate and brackets, use clamps or weights to force the plate into position. You dont need to drill holes in the threshold plate because the spotweld cutter already has done that, but you will need to clean away any coatings from around the holes where you are going to MIG rosette.
Dont try to MIG a site where braze is or you will get a surprise. Brass and zinc burn at the temperatures a MIG generates, and creates nasty fumes you dont want to inhale. Your only options are to totally clean (grind or sand) the metal joints free of all braze and attempt a MIG, or oxyweld. Braze is very difficult to remove completely, and MIG is rather unforgiving in this respect.
Pelican Technical Article:
914 Longitudinal Replacements
Porsche 914 (1970-76)