Jeff,
That's gonna be a hard one Jeff. GM has used the same block castings for different CI motors over the years (what year are yours?) for instance the 3892657 block cast in 1967 is (excuse me) was a 302, a 327, and a 350; the 3914678 block in 68 is the same and this goes on and on. The best thing to do is find your casting numbers as the 307 is unique in the GM lineup and didn't share this multiple CI issue. If you are uninformed as to how to locate the casting numbers this link should help.
http://www.mortec.com/location.htm
Wes