Did you put in a new oil pump when you put this thing together?
Pumps wear out, in that the clearances between the two pump gears and the housing gets too big.
When this happens you get low pressure as you describe.
If the pump is new, the pick up tube may be leaking or the pressure relief valve is stuck open.
BTW the low pressure might have wiped your bearings. If you drop the pan to change the pump, take off a couple of bearing caps and have a quick look at the bearings for signs of damage.