350 low oil pressure
 

1973 Blazer stock 350. My oil pressure is dropping to 3-5 psi at idle after pushing the motor hard. Runs about 40 at cold start and stays about 20 even after engine is warm. Only drops to the danger level after a couple miles of wheeling hard. The truck is strictly off road so I don't really know what the pressure is like during highway driving. Also the top end starts chattering when pressure drops.
I don't really know the history of the engine so I assume it's reaching the end of it's life. I'm just curious if their's any simple things I should be checking before ordering a crate motor.
I've heard a bad distributor can cause a loss in oil pressure but the symptoms I've read about don't seem like mine.

With that age and not knowing how well it was treated early in life, it can be the oil pump pickup screen blocked up to a worn out pump. The chatter is lack of oil to the lifters which could be from either of the a fore mentioned issues or even oil passages, lifters etc. just full of hardened oil residue from lack of oil changes or cheap oil/filters before you got it. Have you ever taken the valve covers or intake off before and seen inside of it?

What I think you're doing is you're pumping all the oil up to the heads and it isnt' draining back to the pan. Your pressure is fine so it isn't the bearings or tollerances, it is lack of oil for the pump to suck up. Maybe look into your fill level or get a deeper pan.