I'm working on a similar idea. The 2 piece seal for a Chrysler 440 engine is pretty close to the diameter of the GMC seal surface. I'm considering milling a flat area on the back of the GMC block and rear main bearing cap, and making a seal holder from aluminum that will occupy the same area as the rope seal does now, then grind the crankshaft to the correct diameter for the seal. The seal holder would attach with flat head screws.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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