I used the Viton crank seal too, but still had oil leaks. There was thread here I think some years ago, about how the Pontiac rear seal cut-out wasn't machined well enough (not round) to use a Viton seal.
There is an old/antique tool, kinda looks like Chinese finger trap that's made for replacing the rear main seal with the crank in place. Never used it, but it may work if you want to use the rope seal. I have replaced a formed rubber seal in the car with crank in place (different kind of car & engine) before. It's a bit of a PITA but it can be done.