Try either Ames Performance or Butler Performance. They both specialize in genuine Pontiac parts and speed equipment for the 301-455 platforms. Seem to recall that there is an insert kit that will convert the rope seal to a neoprene or similar version. It is true that the rope seal is a pain in the *ss and has to be installed just so or it will leak. The builder has to be Anal-OCD when doing the job which is why a lot of shops won't go near it; too much time and patience for too little payback.
U say your using 15w40 motor oil. I hope its not delo 400 or another diesel type oil. Very high detergent and can damage internal engine parts. Get a can of Lucas oil stop leak. Also 20w50 conventional motor oil. Try that. Should at least slow it down.
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