I just measured the seal surface on a 270 GMC crankshaft. It's 2 7/8" (2.875) in diameter, and 3/4" wide. I'd suggest finding a 2 piece crankshaft seal that's close to the right diameter (maybe a big block Chevy V8 or something from MOPAR), grind the GMC crankshaft seal surface to that diameter, and make a 2 piece seal holder that bolts onto the back of the block and the main bearing cap. There's no point in reinventing the wheel when a simple resize of the crankshaft will give you a smooth surface, and a seal that's already been engineered to work with an engine.
Jerry

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