Some engine's rear seal leaks can be reduced or eliminated with a new/better PCV system. Oil pressure should have nothing to do with a rear main leak, but crankcase pressures can. I have heard of people on my Pontiac forum getting very good results with the adjustable Wagner PCV valve...
http://mewagner.com/?p=444
The old Pontiac motors tended to have high crankcase pressures, to begin with, and when they are souped up...it just gets worse. These valves are said to work very well when it comes to high crankcase pressures under WOT blowing the dipstick out and rear main leaks. Pontiac motors had a "stroked" design from the factory and stroked motors tend to build more case pressure. I have not tried them, but they have a high success rate from what I have seen.
Before you click on the Wagner link...be prepared, they are not $5.oo!