Having the big block steering shaft is a good start. People change them but up until the last few years not many people knew there was a difference in them so there is a good chance the column is original. Frame numbers are on the drivers side of the frame toward the rear of the car next to the gas tank. You can take a wire brush and lay down beside the quarter panel and start hunting. Usually not that big of a deal to find unless the frame has a bunch of rust. The frame almost defines it as the real thing. Build sheets might be found but I don't have much luck at it. Did find one on a 70 GS gas tank under the padding on top last winter. Suprisingly it was in good shape. It's a good thing to check the carb and distributor too since many times a donor engine gets those from the original powerplant.
Check on firewall for kickdown cable hole Turbo 350's utilized lower hole for cable, Turbo 400's did not so there would be a plastic insert in hole. Turbo 400 means BB car
