" . . . is the GMC cab and bed a directly fit to the Chevy frame and front clip?"
Yes (the tail gate might confuse people?).

"I've also got a rebuilt 235 sitting in my shop, will it bolt up in the GMC?"
Yes, but it is shorter than the GMC sixes - it will sit further back from the radiator.
But, you referred-to using the Chevy frame?