Dan,
A 76 351 could be either a 351M or 351W. The 'short deck' Clevelands ceased in 74. 75-79 Cleveland-looking engines would actually be 351M, a taller block version of the 351C. It used the same block as the 400.
A 351W is a tall block version of a 302.
Either one would be a 2-barrel, and would have the engine code 'H' so that's no help. You have to know what to look for in either case to know what you're really looking at.
Milo