Oil Pan/Oil Pump ?

[65GUS]

I have a 1965 Plymouth Fury 3 with a poly 318 motor and auto trans. 120k miles,runs great but I'm losing oil pressure, last year went from 49 to 45 at hot hwy speed and 20 to 16 at hot idle. this year hot hwy speed is now 40 lbs and hot idle is 12lbs. looking to see if I can just drop the oil pan with the motor in the car to either check the bearings or install a new oil pump without removing the motor. also is the rear main seal a rope or neoprene seal ? Thanks for any info