I run a 377 stroker in my Beaumont , opposite combo to yours. I run 4:56 gears in my car and it pulls hard right up to 7500 all day no swea I have heard of some 383 motors pulling to 8000+ Rpm .... What total timing are you running on your motor? It sounds like a timing issue to me. I run 36 degrees total timing , but if I run 33 degrees total its flat with no power it loves 36 or 37 degrees. But if you had it on a dyno im sure they set it up.
Kevin