I'm assuming the
oil pump was replaced when the engine was rebuilt. Is the pump a stock replacement or a high volume unit? If the engine has extra bearing clearance put into it, then I'd be checking that it's got a high volume pump. If it's already got the HV pump, then I'd start looking for why the
oil pressure isn't higher than what you have. A HV pump with cold
oil should pull in the 70-80 psi range at moderate idle.
I built an oil pump tester that checks both volume and pressure before installing the pump in an engine. It's been a lifesaver on those pumps that have relief valves that stick either open or closed or the oil pressure just isn't there. The tester works with a variable speed drill and the drill rpms are basically equal to twice the engine rpms so I can simulate the oil flow through the pump at various engine speeds.