AT the risk of repeating myself, if you hit 70's with the same engine, then had a better pump built and put in, by TOM, AND changed the timing gears. Where would be the weakest link?
Am I repeating myself? Dude, check the timing, i'm telling you it's off. Then check the hand hole in the pump, could be a rope in it or something, but I doubt that. Finally, unless you're pushing somewhere in the neighborhood of 600 hp or so, at THAT rpm, the impeller you chose is wrong. But it should still push you well into the 60's.
Ray