;32
Rust - the frame is about the main area. Haven't seen too many with a problem though - just surface stuff.
The 68's differ slightly in the ft guards behind the wheel as from 69 (or 70?) they didn't wrap under as much to avoid stone chips etc. They also had fibre optics for the lights - something I know nothing about.
They only made 17316 of the 70 model, 21801 of the 71 as opposed to 38762 of the 69.
Get yourself a Corvette Black Book which details the production numbers, rpo codes for each model. When you have bought the car, buy the workshop manual for that year at around $150 - a good investment as it details everything!