Just a thought here, my 360 kind of went thru the same stuff with the oil pressure drop off when the engine got warm (operating tempature). It ended up being the clearances for the bearings and rods were set at GM specifications and this screwed up everything in the lower part of the motor. This was due to the machine shop not knowing that AMC specs are very tight down on the crank and the such. Like a dummy, I did not check to make sure that everything was correct. Different story with the 401 that went back into the car. Not wanting to scare anyone here, I would check your clearances and see if they were set by the machine shop to GM specs. A lot of shops do not have the specs for AMC's anymore and instead of asking the customer if they have them, they will use the GM specs so they won't look stupid or something.