I wonder if there's a way to machine a round recess in the back of the block and install a modern day one piece seal into it.
Doable on a Chevy engine, but the GMC flywheel flange is larger diameter than the rear bearing journal from what I've seen here. It would be tough to install a one piece seal, unless you get one that's stretchable. LOL! Here's a pic of the modification on a 216.
