Oil Pressure Dropped...

Had the same thing recently on my rebuilt 340, oil pressure was good for a few days then dropped off.
All my main and rod bearings are shot, crank is in a sorry state, cam bearings gone as well, also had a HV oil pump with stock pan.
All this happened in 350 miles.
Ironically the cam lobes are still in great shape.This engine sat for 3 years after rebuild and in hindsight it was very hard to crank over when I was priming the oil system, 1/2" flex bar but still tough to turn.I have it in pieces but still can't see what caused the problem, I am now guessing it was built too tight, had 0.010" undersize bearings in it, maybe wrong but can't tell as the crank is to bad to get a good measurement.
The oil pump was also shot, if I were you I'd drop the pan and pull one of the bearing caps just in case, only takes a couple of hours as long as your steering isn't seized.