My 1957 GMC...Use a 70's-early 80's Camaro Subframe or a 1981 Monte Carlo SubFrame?

You can use the mounts, brackets, and pulleys off an early 2nd gen Camaro. It's basically the same frame, and they came with a BBC.

 

The differences in a later F-Body (Camaro-Firebird) sub-frame from an earlier one will be in the front of the frame, where the bumper mounts/etc are. Later frames would have different mounts to accommodate the later crash bumpers. Other than that, the frames from any 1970-1981 F-bodies are basically interchangeable. Just be sure to get the proper year/make/model information for ordering steering/brake parts as there were some minor changes.