rear floor pans... notorious for getting wet and rust..

 

Just finished patching floor and rebuilding inner rocker on drivers side of my 68. Big pain. If you find major rust in these areas it should drop the price substantially.

 

Just finished replacing the full length floor pans on both sides and toe board on one side in my '66 Coupe.

I would agree that pulling the carpet back and having a look would be well worth your wild. Would also take a hard look for rust on the underside of the car. Not impossible to fix rusty floor problems but I would sure calculate that into the price I was willing to pay for a car.