oil pressure and knocking

[69SCSS]

I'm grasping at straws here since my motor is sounding like it's got major rod knock, but here goes. I finally drove my truck more than a few minutes on thursday. I went a couple miles and it started to rattle pretty good while I was accellerating, but would not rattle at idle. I noticed the oil pressure had dropped, when before it would be at 40lbs at idle, it was maybe 20. I cautiously continued on and at the next light the oil pressure was about 5-10lbs. The rattling was worse. I got where I was going and it seemed to be running the same, but if you hit the gas it would rattle like a diesel.

So here's my question. If the oil pump failed, I'm guessing it wouldn't be so sudden, unless something actually broke vs. normal wear. Just before all this happened I had pulled the oil filter to see if there was any metal in the filter, since the guy at the tuners noticed the rattle. he thought it was rod knock, and it kinda sounded like that to me too. Since I found no metal in the oil or the filter, I just put a new filter on and refilled the oil. That's when I took the trip on thursday.

After really getting a good listen, it almost sounded like the sound was top end, like the valves were loose, or the lifters were clacking. I'm kinda at a loss here, hoping that my 402 isn't toast! I've had a few strange issues with this motor (including a mechanical advance that retarded instead of advance) which I resolved by swapping in a different HEI.

So if the oil pump failed, would I get no sound at idle, rattling sound at accelleration, and at decel? I am shopping for engines, but would try swapping the oil pump if that could possibly be it. I mean at this point, what do I have to lose? just the money for the pump and the gaskets, right?