I was always a 2nd Gen guy, you know, a massive steel frame under the car. However, I noticed the frame on my 3rd Gent GTA is kinda meshed into the floor-pan. We need to replace the current floor-pans due to rust-through. Are there any special procedures since the frame is different? Or do we simply weld-in..........

 

First drill out all the spot-welds so you can leave the stamped-sheetmetal subframe in place. Dunno what pans you bought, but it's best to finish either the right or left half before starting on the other. Even better would be welding in subframe connectors before starting on the pans.

 

why would it be better to weld the sfc before tackling the floorpan?

 

Just to add some strength to the body before you cut out a major portion of its structure. Do that or remove as much weight as possible and/or brace/rack the frame so the body does not bend. I know of people that just cut+weld, but ideally it would be best not to rely on the floppy thirdgen chassis to retain its shape with the floor missing.