[84LX89GT]

I agree with the previous threads, something is causing the oil pumps to lock up. Something i would do is after the driveshaft twists, take out the bad one, pull out the shaft piece inside the pump, put a new driveshaft in and find a socket that fits it, then manually turn the pump with a ratchet to see if it's bound up - make sure to attempt to turn the way the distributor gear turns, then the other way and back again.
I had a '73 F350 that ran for a month with no oil pressure that was 5 quarts overfull (bought a few days before from a friend for $100) and the pump had junk in it, and had twisted the driveshaft. A new driveshaft fixed the problem and i sold it for $400.
Chances are some crap got into the pump or the pickup is restricted. Also on higher mileage engines it's easy for silicone or pieces of coked oil (carbon) to fill up the pickup or get into the pump.
Maybe the first oil pump just wore out and some junk in the pan, or stuff that fell in while installing got into the new one?
Anyway, good luck.