David - was it running 20psi when you rev'd it, or was that at idle? If at idle that's about what mine sees, I wouldn't worry too much - as long as you're getting around 60psi when you rev it to 2k or more. Also, are the lifters ticking? If it has been sitting for that long they may be dry; you'll need to run the motor for a few minutes at an elevated rpm to get them pumped back up. I don't think that will affect oil pressure, but maybe? I would definitely start with the gauge, and see if that makes a difference. Is the factory gauge still hooked up...or did we get an idiot light? I can't remember off the top of my head, I haven't looked at them in years.

The pump is pretty easy to change - just make sure you mark where the rotor is before you pull the distributor, and where the slot in the end of the pump shaft is in relation to the distributor; you'll need to get it back into the same position when you replace the pump. It sometimes takes a few tries, as the shaft turns a little when it engages the drive gear on the crankshaft.