you. they are different but you are correct they are an easy enough retro-fit but some people just want to bolt it in and go which is why Mancini (and others I'm sure) have different listings, pricing and part #'s.
FYI, the only difference is that the drivers side mounting bosses on the 360 block is 1/2 inch narrower than a 318. Easily fixed with a 1/2 inch spacer and you can use the 318 mounts. That said, you do not have to run the spacer, you can simply bolt the bracket to the boss from the opposite side and be just fine...