72 318 Scamp. Low oil pressure 1 minute after startup.

Hi everyone. I recently bought a 1972 Plymouth Scamp with a 318 and 904 automatic. The engine/Transmission has to come out because of a broken flex plate.

I have a question. The engine starts up fine with no noises besides the flex plate scraping... ok I didn't run it for long. When the engine first started it has 50 to 60 psi oil pressure. Then about a minute later the oil pressure drops to about 5 psi! Any suggestions on what to look for when I take the engine apart? I did put new oil and filter in it after towing it home and the oil pressure was still the same. Going to try and pull it out this weekend for inspection. Any help would be appreciated.

P.S. Sorry for the dumb question. I have worked on a lot of Chevys but not a lot of Mopars so I am learning.

Thanks

Eric