I crawled under a 1970, 1973, and 1974 Challenger today.
The 1970 has never been touched/modified in the area where exhaust hangers might have been. It is a 383 car. It has some sort of stamped metal piece on the passenger side only. I can't see how that piece relates to an exhaust hanger,
The 1973-74 cars had no evidence of ever having any kind of factory exhaust hanger.
so I guess I will weld up all the holes from aftermarket rubber hangers, and place whatever hanger I need to get the job done.