My rear main on a 76 Slant leak immediately after rebuild. The rebuild was performed by an experience builder although he did not stagger the seal with the gland. I installed the one piece seal from fastfish in the manner that Slantsixdan stated. Over 500 miles later still no leak.
I hope the result continues but unfortunately 500 miles doesn't mean much.. Having worked in the trade for many years I've seen to many seals that work fine for a thousand miles then start puking oil...
Best thing for rear main seals was when manufacturers quit putting a big flange on the back of the crank and started using a full circle one piece seal...