I'd drop the pan, clean it out, and replace the oil pump and screen. Oil pumps do wear out, and your motor is 37 years old. That would be a good starting point, then you will know if it is something internal like excess clearances, etc.

I've always had a policy of pulling the pan and putting in a new oil pump anytime I use an engine from a wreck or junk yard. The one time I didn't do it was when it bit me.

Oh, and welcome to the forum.

Don