Some ideas
You never said anything about crank bearings as part of the rebuild or why this engine had low pressure to begin with. The assumption is that it's a bad pump. But what if it's not the pump.
The drill won't provide near the oil pressure as the actual engine does. Plus, the vasoline trick is thick. That's why it primes good. But the thicker (than oil) vasoline should give an artificially high oil pressure reading. So once the vasoline is melted/absorbed into the oil itself, I'd expect the pressure will drop. And drop even more as the motor warms up and the oil thins out. So, at idle, maybe 10 to 15 psi, warm, is OK based upon old bearings, rings, etc etc etc...
Regarding why it ran hot, or if it's hot at all.
It could be that you've got a bad thermostat. I've had engines with stuck shut thermostats that'll boil the coolant in the radiator in no time at all due to the motor not 'flowing' the coolant thru it. Could be something simple like this
It could be that you've got a bad headgasket / and-or / a cracked head causing compression leakage into the coolant jacket, causing a 'boiling' type effect
It could be that you pieced together the cooling system with old parts and the clutch fan isn't locking shut and the fan is spinning too slow to cool.
It could be that you've pieced together the cooling system and the fan is a reverse direction fan (been there-done that) and not pulling air thru the radiator but instead just making a lot of noise and moving no air.
It could be an old radiator that just doesn't flow.
