Oil leak

I have a '77 Excalibur. 604 cid. Reassembled the motor after a oil line break last year. Paid a lot of attention to sealing rear main because it seemed to have a light oil spray last year. Started it no leaks. Ran it about 20 minutes and the oil spray returned. Boat never had a oil cooler and I think it is getting too hot. New, motor had 60 psi oil at idle and 75 when revved. Hot has 30 at idle and 60 revved. Is this about right. Full synthetic oil and mark IV rear main. After I add the cooler, do you think rear seal is bad after couple hours of run time or their another problem. I am still going to add cooler for safety.