The H-M "flywheel" is actually the front part of the fluid coupling, and I'm not sure if GMC and Chevy crankshaft hub flanges have the same bolt pattern and/or size. I do know the design of the starter ring gear is slightly different. That might become a sticking point during a swap. You'll also need to relocate the front motor mount because the Chevy engine is about 2" shorter than the GMC. Where is the block cracked, and how badly? If it's not in an area exposed to combustion gases or cylinder pressures, it might be better to explore the possibility of getting the crack repaired. I think the dial-indicated adapter ring between the engine and trans was a bit of overkill- - - -the Hydra-Matic division of General Motors was a picky bunch back in the 1950's!
Jerry
