After I stripped away more of the primer on the floor, I found more rust to deal with. I hate rust.
I decided to remove the floor pan in one piece to work on. Oddly, the driver's side footwell area was is pretty decent shape so I left the in place. Removing the floor from an 850 coupe isn't all that hard, much easier than the floor in a 600.
I used the floor out of the donor body as is because it was better than this one. Still had lots of rust patches needed tho. I had the floor on a couple of sawhorses which was a lot easier than attempting to do it if it was still in the car. Photo below shows all the patch pieces welded in place and ready for some grinding and sanding. The large patch piece is from the 1300. I ended up making one good floor out of 3 floors plus some new sheet metal. The rear seat footwell area was pretty bad and I made most of that up from scratch from 20 gauge steel. Getting the floor back to rust-free shape definitely took a LOT of hours.
Here's the floor after sanding down all the welds.
Here's the repaired floor almost ready to re-install. What looks like surface rust is just stains from the muriatic acid I used to remove some stubborn rust. I will media blast the floor top and bottom once all metal work on the body is finished.
The last patch piece for the floor is this piece that goes against the right side front fender well. A bit tricky with the compound curve in it.
Before I put the floor back in, I need to replace the rusty heater pipes that run through the tunnel. The ones out of the donor car are in great shape except for some internal corrosion that I'll need to clean out. When the floor is fully welded in, I need to hammer out some "humps" either side of the tunnel at the forward end. Abarth did that to make clearance for the coolant pipes. Won't be too hard to replicate because they beat the humps out pretty roughly at the factory.
If I forgot to mention it, I hate rust.