Rotax,
Suggest you get a copy of Barry R's book, which can be ordered from the Survival Motorsports website. It goes thru a number of ways to identify FEs... The drill bit test is a well accepted way to determine original displacement, for example. The C2 or C3 engineering numbers may not mean anything specific, but may help you identify by exclusion. Being its a solid lifter block, it could possibly be a 390 PI, or even a 352 or 390 Hi-Po...
Good luck!
Bruce