Motor Mount modifications for 340/360 into Early A

I don't know anything about early A-bodies but if you have one with a 273 why do you have to do anything to the passenger side to put a 340/360 in?

I know the ears on the drivers side of a 340/360 block are closer together than the 273/318 but the passenger side of the block is the same but that can be handled with a simple spacer.

Curious minds want to know! :)