How to tell a 318 from a 360?

There is a casting number on the side of the block behind the starter that will look something like this XXXXXXXX-318-XX. The displacement is the 3 numbers between the 2 dashes, i.e. 273, 318, 340, 360. Chances are you have a 318 since they were far from gutless, even in the '70s. Hope this helps.