I did that after speaking to another member that did it (but provided zero pictures of his install). It’s not as easy as that. The rear crossmember has to be modified heavily to fit the 68-70 B body tank. As in, an entirely new section of crossmember has to be welded in. The B-body straps are also quite different, I lengthened my A-body straps.
I’d file that one under “can be done”, but it’s not as nearly as easy as just welding in a flat piece of sheet metal and strapping in the B-body tank.
