First things first!! Most excessive rear main seal leaks are caused by BLOWBY! If the pcv is partially or completely clogged the pressure cannot get into the intake system and instead pushes oil out the seal!! First thing to do is to clean all the crankcase vent tubing and replace the pcv with a new one.
ONLY then should one be thinking about repairing the leak.
I always use a stock oil pump, the high volume ones are for much higher rpm and stress than our little engines will ever see..
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />