A Chevy truck would be easier to restore and maintain because parts are readily available. The GMC has a lot of unique GMC only mechanical parts. They had their own engine,driveline and brakes etc. You might have trouble finding parts.
You would put the gauge in the hole in the block where your present oil gauge is hooked up. The hole is probably down low on the block and not far from the oil pump.
This strikes me as a wrong parts combination. Wrong pistons or heads or just an incompatible combination. A wrong timing gear or camshaft. Maybe one of the offset keyed racing timing gearsets that has been installed wrong .