The M-45 pump won't clear the oil pan. That's why the later model engines have a dogleg in the passenger's side oil pan rail and a bulge in the pan. I'd be surprised if the GMC pump will fit, as it's considerably larger than the M-45. There is a way, with a LOT of precision machining, to fit a set of Chevy V8 gears to a 216 pump.
What weight oil are you running? It's virtually impossible to maintain pressure with the water-thin oils some people insist on using for some odd reason. Any "10-something" oil will drop the pressure noticeably- - - -it's just too thin to work with the loose clearances and low pump volume most stovebolts have. The "5W" oils are even worse. A lot of us run 15W-40 oil and see much better oil pressure, particularly with a hot engine.
Jerry
