AS I mentioned earlier, I would recommend you drop the pan and look around because you will worry until you find the trouble. The only thing that concerns me is the knock. When I first start a fresh GMC, I don't even look at the gauge. I leave the valve cover off and if the rocker overflow is flowing, I figure the rest of the engine has oil. I'm one of the old geezers that adjust the valves with the motor running when it is hot. Even new oil pressure gauges give false readings. Our Cummins engines are programmed to show a stop engine light for a few seconds and then kill the engine. I keep pressure sending switches on hand because they fail quite often. I also keep two master gauges to check oil pressure. Your original gauge probably wasn't very good and the after market one may well come from China.
You say the rear main leaked. Did you use a rope seal or did you use a National type seal. I use the old rope seals. I kind of pack the ends with a drift punch and then trim the ends. If you leave loose threads, it will get between the cap and the block and you will have too much bearing clearance. But I wouldn't worry that you spun the rear main. It takes a lot of clearance for it to spin. If you do need to replace the rope, Lisle makes a gadget called Sneaky Pete. They are less than $10 and they work. Starving for oil usually shows up on the number one rod bearing because it is the last one to get oil.
Is the machinist you used familiar with valve guide to valve clearance? These old machines require a lot more clearance than newer engines. Too tight can cause valves to hang open which causes a miss and a knock. Did you bore the block. Unless you bored the block to oversize, I wouldn't recommend chrome rings. They just won't seat with just a hone job. Unless you bored it, I would use cast iron rings.
I'm kind of the opinion that people worry too much about oil pressure. It's not the pressure that lubricates, its merely having oil between two objects that rub together. Until the mid 70's, Cummins engines ran between 50 and 60 lbs under load. I think it was in 1976 they went to a 30 lb relief valve. It took some getting used to, but now it is just a way of life looking at 25 to 30 psi on the oil gauge. It takes a couple of extra hp to go from 30 to 60 psi. And HP uses fuel. I built my first GMC engine and drove a 1950 GMC to school in 1957. I finally got a drivers license in 1959. I've been tinkering with GMC ever since. I have over a dozen pre 1959. They don't have much oil pressure at an idle. In fact too much oil pressure alarms me more. It is a good sign of coolant in the oil.
