I checked on my '53 with stock rear bumper and the tops of my pipes are 5" below a line stretched across the bottom of the end of the frame rails. They clear the bottom of the bumper by about half an inch. Not sure if 54-55 are the same bumper brackets.
I agree with Tony that the bumper on first would be a good idea. You could stop at the mufflers until the bumper is on before adding the tail pipes and adjusting the hangers. - Pete

Doug Peterson
53 Chevy Pickup