Carl.  Glad you figured out the rear main seal leak.  Don’t feel too bad about getting that lower seal half in backwards as that’s extremely easy to do when performing that operation under the vehicle with everything being looked at upsidedown.

As far as the oil pressure goes, now that the engine is out do some preliminary connecting rod bearing and main bearing oil clearance checks.  Give some feedback as to the weight of the oil that was being used and the brand of the oil filter being used.  Was the crankshaft freshly turned during the last rebuild or just polished?  Simply polishing a crankshaft and reinstalling it gives a reduced oil pressure over being freshly turned to specification to that’s also a consideration.  The cam bearings can also be a source for oil pressure loss and especially if the engine was not completely rebuilt during the last tear down.  And as already been mentioned, the oil pump is always high on the suspect list but check for a crack in the oil pickup tube and adequate seating of the rubber bushing for the tube going into the inlet side of the pump.