Is it going in that cool Challenger? If so, my answer would be simple. 383. I've always thought e-bodies were too hefty to be small block powered. Obviously that isn't a blanket statement. But if you have the 383 already it's a lot less legwork. Unless you enjoy the chase, as well.
I like 383's and have had good luck with them. Haven't done the Magnum in a car, but the 360 in my truck didn't have the low end grunt my 400 (not a 383, but a low deck ) had. Same gear, roughly the same tire size.
That probably helps in no way. But just my