I might go out soon and look at a 1970, 4spd, 350, ETC. It looks all right on the outside but what should I look for really? Floors, Rust, What?

-Thanks

All the usual plus windshield pillar rust and rear frame rail rust.

Rear frame rail rust is the biggest thing. Then floor pan rust (where the floor meets the firewall) and in the under rear seat area. This stuff is not available repro and you will have to make your own patches.

i don't know how different they are from 70, but both of my 79s had rust between tank and rear bumper. also floors rusted above front sub-frame. one car had 55000mi. and the other was 120000

They are bad where the rear lower tail panel comes together with the inner rear pan. That section where the rear frame rail meets the rear inner tail panel.