Hi,
Yes, I agree that a spot weld drill will be very useful. Also, in general use a cutting wheel to carefully remove much of the old floor. Most saws or cutting torches would very likely do damage to the frame underneath, and/or the fuel or brake lines. Speaking of which, you might already know to get under there and carefully locate all those lines before doing any cutting, i.e. where you'll need to be especially careful.
Yes, Clecos or pop rivets or even sheet metal screws can be used to temporarily hold the panel in place until you start tack or plug welding it in place. I think the overlap will be obvious, once you get the old tack welds separated. Just use the cutting wheel to "rough cut" the old floor out, leaving an inch or two extra at the perimeter. That remainder will be removed by drilling and grinding loose the factory tack welds.
I'm afraid I have to disagree a bit on a couple of the other points. Yes, the bodies were built separate from the frames originally (but each were undoubtedly built on carefully designed jigs). I bet many folks over the years have managed to install the floors without bracing the doors, perhaps with the engine and gearbox out of the car, too.
However, knowing that the combined body and frame of the IRS cars tends to flex a little. That's even more likely considering the car's age now, plus that the the body and frame have surely "settled" together a bit as a unit over the years.
So, if it were me, I'd prefer to have the car close to it's "normal" weight and sitting on its suspension while installing the new floors. I'd wait to pull the engine and gearbox after the floors are in.
You'll be removing the interior anyway, definitely including the dash-to-floor yoke or brace and likely also the steering column. By then, it would be relatively easy to go ahead and remove the rest of the dash and heater so they are completely out of your way (the wiring harness might be disconnected, marked and taped, zip tied or wired back, but not completely removed). The pedal assembly is relatively easily removed, too.
My point is, by the time the dash is removed, it's relatively easy to bolt in a brace from the windshield frame bracket/A-post to the hoodstick bracket mount at the B-post. So why not just add this extra measure of security? Sure, you could *probably* get away without bracing the top of the door. But why take a chance?
I just think it would really be the pits to weld in the nice new floor and *then* find something had shifted, that the the body had developed some sort of twist!
Welded-in door braces are often used effectively on full body-off-frame restorations, but I think bolted-in braces can work equally well when done right. I'd want to design any braces so they could be fitted before the door is removed and to allow the door to be easily test-fitted from time to time during the floor installation.
In my opinoin, it's better to be overly cautious than to have to do the whole job over again. Like so many projects, you might spend 75 or 85% of time and effort on preparation and only 15 to 25% on actually doing the repair!
Heck, I might even cross-brace to the opposite hoodstick bracket mounting holes and bring a another, vertical brace from that to the driveshaft tunnel/parcel shelf, too.
When it's finally time to install the floors, you probably know to go slow, tack weld the floor in just enough spots around the perimeter to keep it in place. Then refit the door to be sure nothing has moved. Only then go back and finish welding the floor a little at a time, not concentrating a lot of heat in any one area, to prevent warpage.
Incidentally, while you have so much of the interior stripped out is an excellent opportunity to make sure the lower steering column bracket is in good shape, correct any problems and perhaps even reinforce it if it appears fine. I don't know if this bracket was ever improved on the later cars, but it's very common for it to crack along the 90 degree bends on the earlier cars.
All that's really needed is a second plate of steel wrapping it and doubling up its strength. This can be welded in with relatively little fuss or damage to the existing paint on the firewall, if done carefully. It's a bugger to get to this bracket and repair it if it breaks later once the car is all back together. If that bracket ever breaks and needs to be repaired in the future, *a lot* has to be removed (which of you've already done for this floor project).
Yes, the top side of the floors can be pretty easily painted and touched up (don't forget seam sealer). Heck, it's all covered by interior trim and carpet, anyway, so doesn't even need to be "perfect". It will be a bit trickier to touch up the paint on the underside, which is probably even more important. But, it can be done.
There are various rubberized pads at all the body-to-frame bolted mounting points, that will need to be removed before welding in the floor. These are used to level the body on the frame and there can be varying amounts used in each location. I always try to remember to count the number of shims removed from each location. But, even then, if replacing some or all of them with new ones, the replacements can vary in thickness and it's common to need to do some fiddling to get it just right. The rubber shims on the outriggers, in particular, effect door gap front and rear.
One more thing that might be done at this time, if you wish, would be to drill a few holes and spray some sort of rust preventative inside the rockers. You'll have relatively easy access while the car is apart. There are already some drain holes that might be used on the inside below the floor level, just check that those aren't plugged up in any way. The most common place to drill additional holes is along the inner rocker above the floor level. These holes which are plugged with plastic caps after the rust preventative is sprayed in, and in turn covered by carpet trim eventually.
Keep us posted on your Winter project(s)!