Looking at an E body 70 challenger

Yeah, I would keep the 15's as well. Mine came with 14's But, I will probably change them out.
You are going to have a heart attack when you see what original 15" ralley wheels are going for, the last set I seen for sale was $800.
My Challenger also came with 14", which are still on the car, but I got lucky and my cousin sold me a set of 15" cheap...He did not know what he had.