Floors and cab corners are pretty much always shot on these old trucks, especially for you northern folk. Also look at the lower cowl sections on the "new" cab. Anywhere two panels come together, there is always a chance of rust. The sheet metal may be 99% rust free, but the areas where everything comes together are the important parts.
If the "new" cab is in better shape, I would say go for it. It would more than make up for the time it takes to hack and fix an old cab. Plus, you have a better chance of things lining back up the way they should with a complete cab.
If the front end of the GMC is in good shape, you could always sell the fenders, hood, and grille. The GMC guys are always looking for good parts. You may even be able to make your money back.
