The Chevy 4000 and 6000 series truck fenders, hood and grill are a little different, and mostly a little bigger and wider. Wheel openings are a lot bigger. The cabs are interchangable so the only question would be what do you need to do in the raditor support area if you are swapping.
The big truck bumpers are different, so the valance is probably different too. The bumper brackets are completely different and I doubt swapping little truck to big truck bumpers is a bolt-on project.
GMC made trucks, the 280 to 350 I think, that are the same size as the Chevy 4000 and 6000. They also made larger trucks such as the 450 series that have even bigger fenders.
I'm not an expert so someone else would need to offer more detail.
