John caught the question I was going to ask. When I use a stock oil pump I do not tighten the centering bolt tight. I let the oil pump "float" a small amount. This is on GMC's.
This can also make the HEI not drop in low enough if you are not doing something with the stock flange. I have machined them of and made my own clamp which allowed me to lower the HEI into the pump. On the one which have the pined "extention" I have welded additional material on the the length and heat treated it. None of those have ever failed...Good Luck
