The guy I got my '70 Challenger from had another '70 Challenger that he has been fixing up. It was born with a slant six and I told him he should really leave it in.
Call me crazy but I'd walk past a lot of other Mopars to see an original slant Challenger. There's almost none left. Just like slant six Chargers. He already has a sweet '70 road runner with 383 power.