Back in the good old days, the "serious" chevy racing engines used GMC oil pumps. I've never seen any real data on the pump volume vs. pressure characteristics, but based on the gear dimensions the GMC pump might produce about 20% more volume than the Chevy pump. Probably not enough to fix your low oil pressure or to justify the oil pan modifications needed to use the GMC pump in the Chevy.
Tom Langdon, among others, says that there is no need, as the stock Chevy pump has plenty of capacity.
As Jerry says, the low oil pressure problem is probably excess clearance in some or all of the bearings. Or, it could be due to lack of a restrictor orifice in the bypass filter line. Plugging the filter inlet line (if it is a bypass) will show if that is a problem. Did the low oil pressure readings come on suddenly or gradually? The
specification documents call for 30 psi at 1200 engine RPM.
