Unfortunately, the GMC engine can't be fitted with a full circle lip seal. The flywheel flange is bigger than the seal surface. The Chevy engine has a straight rear flange, so the block can be machined for the lip seal. I've had some degree of success with adding a shim between the rope seal and the block, putting more pressure on the top side of the crankshaft. The same thing can be done with the lower rope- - - -a thin piece of brass shim stock, etc. If the engine is in the vehicle, it's usually possible to lower the crankshaft a few thousandths by loosening all the main bearing bolts and tapping a piece of the thin stainless steel that holds a windshield wiper together between the rope and the block. Then S-L-O-W-L-Y torque the rear main cap a few pounds at a time while rocking the crankshaft back and forth to squeeze the rope into the proper contact with the crankshaft.

If the seal surface on the crank shows any roughness at all, you're probably fighting a losing battle.
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!