Might want to check to see if the bolt pattern on the back of the block is the same. Admittedly the two engines really do look a lot alike otherwise.
Quick way to identify the larger engine is to look for the engine serial number pad by the distributor... in back of the distributor is the big engine, in front of the distributor is the little engine.
There is also a chart somewhere on this site that will help you decipher serial numbers. Might be useful to determine what vintage engine you are looking at.