IT SHOULD NOT BE SCREAMING AT 55....I had a 86 Allegro /454 with 456 rear end...pretty much same as yours....so Iam familiar, sounds like you are in 2nd gear?....even with a 513 rear gear it wouldn't do that in 3rd....You need a tac to see what your RPMS are....that's the first thing..you cant analyze this problem without it....maybe you are hearing something else, manifold leaks, loud exhaust ,who knows? RPMS at 55 first. And if that 454 is struggling that hard , Id say something else may be wrong.....like timing, carb problems,etc. I would not trust it till its fixed.