The only purpose of raising the tranny would be to clear the frame of some other type that the engine is put into.
There's no advantage of any kind, other than possibly a slightly lower center of gravity, which would be questionable anyway.
I'm not saying it wouldn't be done, or can't be done, but the Max Hazan bike doesn't have that done to it, because we were involved in that build and we know what was done with it.