Jimred,
I had the same problem about two months ago with my 76. I looked around e-bay and other auction sites and the best price I could find for used floor pans were $125.00 per side. I contacted a local corvette guru and he told me that in order to replace the floor pans, it would be a major project. He told me that he could fix each side for $200.00 total. His trick was to use 16 oz. fiberglass.
Two days later and $200.00 less, I got my car back and everything looks great. Re-installed the new carpet and you cannot even tell.
Remember, ‘75 was the first year they went to metal floor pans due to the catalytic converter. If the metal under your passenger seat is in decent, fiberglass could be an option.
Just my two cents.