Auto body repair & detailing: Floor Panel Replacement, rubber body mounts, quality panel

Question
I am doing a frame off restore of a '65 GTO. The drivers side floor has significant rust on the front section and the rear (just behind the seat). I have a full panel replacement and was wondering, do I just cut out the rusty stop and trim the replacement panel to fit and weld. Or, should I remove the bulk of the original floor and just replace the whole floor pan? It seems like the original metal is thicker and I've seen numerous posts saying to keep as much original metal (provided its in good shape) as possible. Any help here would be greatly appreciated.

Answer
It goes to asthetics. If you don't want it to look patched up from underneath when it's on a hoist, you need to run the panels to at least the floor joists (supports) and lap the panel over the top. Problem is, the floor supports are where they rust out. i've owned several 65 and 66 goats, i'm totally familiar with their problem spots. You might want to go to a higher quality panel if one is available. Check year one. Figure on having to replace the floor support behind the seat as well- there is a frame mount near the seat belt mounting bolt- they rust out there also. You need to check and see if the existing mounts are in good shape, and if they need replaced before tackling the floor. I would guess that all the rubber body mounts need replaced- especially the 2 behind the rear wheels. These two also support the trunk floor, and will rust extensively. Or, you can just scab a piece over the problem spots and live with the seams. If it's a true GTO and not a clone, it would be foolish to not do this properly. any serious buyer will have it put on a hoist and look for shoddy repairs- and scabbing patches is just that. Bill