Carlos I know the situation you are in. My car is 09C. Car was raced from new according to what I can find out. Engine and transmission gone but the original 3.73 axle is there.
Shipped to a high Performance dealership in Chicago.
Contacts with previous owners confirm in had a “stacks” type SS hood.
All of the “bolt on” 396 stuff is there.
M21 in place, but from a 68 Chevelle.
3.73 axle was not the standard ratio or performance ratio for 325 hp or 350 hp. It was for the L78.
Car was restored in the 90s with all new brake lines and fuel lines. GM panels were used for anything that needed work.
So I know almost exactly the situation you face. The M21 really isn’t a clue without documentation because a gear set can be changed in a few hours (or less).
Document the clues you have. But without original paperwork, there will be no solid evidence.
This table may help.
http://www.camaros.org/diffs68.shtmlAnd yes if you would like me to post pics, I can.
