It looks like a 2-piece neoprene seal for a big block Chevy V8 can be adapted to the GMC sixes- - - -228 through 302. It will require machining of the block and rear main cap, somewhat similar to the way a Chevy six is done, and fabrication of a seal holder from either steel or aluminum. Then make a small change in the seal surface diameter on the crankshaft. This looks like a fairly straightforward operation for any engine machinist with a few brain cells to rub together!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!