I have an all original body and paint 74 Challenger. Have had it since 83.
Base model 318 car.
Original exhaust was already changed to headers when I purchased it.
There is a witness of a similar sized bracket on passenger side frame rail. That said, I most likely had the same bracket you have pictured. Same bolt thread on drivers side rail but never used. It is covered in undercoat.
Always thought the 2 small bolts that protrude thru the trunk (attached to a plate on the trunk inside) were insufficient for a exhaust pipe hanger. I suspect now they are a mount for the heat shield. With the hanger mounted to the frame.
Mike