Other than a few of the basic parts like crankshafts, heads, blocks, camshafts, and sheet metal, consumables like bearings, rods, pistons, and other stuff can be substituted from different engines. All it takes is a machinist who is willing to measure, modify, and substitute non-standard parts. Just today, I picked up a 235 crankshaft that I had modified to run big block Chevy V8 rods and rod bearings. That one's going into a 261 block with pistons for a 292 Ford V8 and valve lifters for a big block MOPAR engine. The engine will have a 4" stroke, 275 cubic inches, and H beam connecting rods rated for 1K+ horsepower. I'll have less money invested in it than if I bought all original stovebolt parts.

There's no reason the same approach can't be made to building a GMC engine.
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!