Unfortunately, this is a common problem among Pontiac V8s of the era. When I was rebuilding my 389, I purchased a Viton rear main seal. They are available from our host, as well as BOP Engineering and other places.
Better than the old rope seals and it can be installed with or without removing the crankshaft. You'll still have to remove the oil pan and probably transmission, but at least it saves you having to remove the crank.
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