[raycow]

It's no problem to use a disc that is smaller than the pressure plate. The problem is that you may run into trouble on the hub protrusion. The GMC disc is one of a kind, and Chevy discs don't normally work for replacement. You might have to machine the hub shorter or have a rebuilder make a custom disc for you. This will all make a lot more sense when you compare the two discs side by side.

Ray