that is a hard spot to see. Some general guide lines are 4bbl carb = 392, 2bbl carb and dip stick mounted by bottom hole = 345 or by top hole = 304.
dang it Jeff, I got that one wrong again. Thanks for the correction. Any way this is just a general guide line because a po can mix and match a lot of different parts when building an engine. My 345 has a 392 4bbl manifold on it.
also I read somewhere that the horsepower that is on the door tag, can tell which motor. Any truth about that