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Welcome to the forum and to Corvette ownership! Floor rust which requires major surgery is often accompanied (possibly caused by) Leaks in the windshield frame and/or t-tops. Do a search here on birdcage rust and you'll find a wealth of reading. If possible post some pictures of the floors with seats/carpet out, there are excellent craftspeople here who could recommend the easiest/best way to approach your specific repair. If you can get under the car, inspect your frame and body mounts closely for rust mites as well. Your avatar does not include your location... are you from the "rust belt"?
Road salt both locales... not a good sign if the car was driven beyond the "season". I do recommend a close look underneath, and at the 'cage. Like most 30 year old autos, these cars can show their age underneath the "makeup".
Shouldn't need to remove the body.
When it comes time to weld let me offer this advice, having just done floorpans for the first time in my life. Set the welder to almost the lowest setting possible. And use gas shielded wire, no fluxcore.
The birdcage is the steel (from late 1975 up) floor, door sill and windshield frame which comprises the "core" of your car's body. All the fiberglass bits are bonded to this core, hence the importance of the core's integrity to ensure the strength and safety of your body structure. To search this subject, scroll to the top of this page. Left click in the triangle just to the right of the word "search", select to search just this catagory and input the single word "birdcage". Read it all... we'll see you after the new year!
Agree, need to find the source of the water and fix. Could be the windshield frame. Pull the trim pieces on the inside and check for rust.
Anyway, I had a few holes in my 76 metal floorpan and quite a bit of surface rust. The area around the formed in nut for the seatbelts was still good though. Rather than buy a couple of new floorpans to install for $250/piece. I decided to wire brush the metal, paint with a rust converter, then paint with derusto, then install fiberglass fabric/fiberglass gel over the whole floor pan on the inside. Tape on the outside kept the fiberglass repair from dripping out. Afterwards go back and spray undercoating on the bottom of the pan. I think this repair might hold up a good while. Key I think is to get the rust treated and sealed. And I think you need to have enough metal floor pan left to provide some structual integrity to work with. Good luck!
Once you dig in and find the extent of the damage you might not think it was a good deal. In some cases money wise its not a good deal even if they paid you to take the car. However the experience and fun of doing the resto you can't really put a price on. Good Luck
what size rivets did you use to fasten the floor pans?
Will rivets alone hold the floor pans? I know they must be steel, how about galvanized
I used 3/16" Stainless Steel rivets. I have a pneumatic reivet gun (They are about $50 from Harbor Freight if you have an air compressor) because stainless rivets with a regular hand rivet gun is a PITA.
I suppose it can be riveted on the inside. But keep in mind it's hanging there with all that weight (yours, passenger, seats, etc.). I would weld the inside and outside and only rivet the front where it meets the fiberglass. Can it be riveted only, yes. If you put a series of stainless rivets in it perhaps it will be strong enough.
Getting the new one in is easier then getting the old one out. Using a ROTO-BROOCH hole saw, a Sawzall, and a Big-azz hammer and chisel help alot!.