WOW, now this urban legend has progressed from headers to crossmembers? There has NEVER been an "offset engine" in any Cutlass Supreme. Most of the header manufacturers at one time or another has claimed this for headers, but I never heard of it for crossmembers. Either way, it's total BS. Think about it. All G-body frames are the same. Many G-bodies other than Olds used the Olds 307 motor. Are we supposed to believe that the Cutlass Supreme (and not the identical RWD Cutlass wagon) used different motor mounts, exhaust pipes, crossmember, linkage, etc? Sorry, but that's not how GM works. Get the crossmember, it will work fine.
By the way, what is it about Cutlii that aftermarket vendors seem to think the cars are different from other A/G body cars? For example, Stainless Steel Brake Corp charges $100 extra for their Cutlass disc brake conversion kits as compared to Chevelle. Needless to say, I won't be buying from them.