Buck Sergeant

FoxbodyKid, I like your plan of attack. You are right about removing the heads last. Your approach of eliminating all other possibilities before removing the heads is the correct approach. I think there is a total of five oil galley plugs, someone correct me if I am wrong. These plugs are best to be threaded and not pressed. And yes, a missing oil galley plug will cause a drop in oil pressure and cause the symptoms you have outlined. Also, a missing oil galley plug can seriously damage the "lower/bottom end." This might help. The back 3 oil galley plugs on the back of the block in the bellhousing area are screw in, the 3 behind the timing chain are press in on a 302. My mistake about the five plugs.

wbrockstar

There is also a plug at the top rear of the block,but its partially covered by the lower intake.If this plug was missing,your pressure would probably be 0 at idle and you would definately have oil leaking at the back of the intake/block.
Have you tried removing a valve cover then priming the oil pump to see how well oil is flowing at the pushrods and rockers?? A oil priming tool is available or you can use 2 long 1/4" drive extensions,1/4 or 5/16 socket and a drill.Be sure to add electrical tape around each connection point,so the socket and extensions wont come apart and fall into the oil pan.The drill gets put in the reverse direction for priming.Mark the distributor housing and the block then the rotor and distributor cap,so the distributor can be reinstalled in its original position.
I doubt its the problem,but is there any chance the oil sending unit might be bad?? If a test gauge is available for rental,you could check pressure with it to verify the sender is ok.
Make sure the ground strap is tight & corrosion free,between the cylinder head (rear) and the firewall.Its the ground for the gauges.One hint of a bad ground here is if the temp gauge reading increases when you turn on accessories like the headlights,heat/ac fan,defrost.
Was a crank kit installed or was your original crank turned with oversize bearings installed?? Is there any possibility the wrong size bearings were installed with the crank used?? If bearing clearances are too large,oil pressure will drop once the engine heats up because of the metal expanding and the oil thinning out.A thicker weight oil can help increase pressure a little,but it isnt a fix.