Not certain of the GMC/Chevy differences, but a 47/48 chevy cab uses a 3 point cab mount and framerail mount fuel tank, whereas the 49+ chevy cab uses a 4 point cab mount and fuel tank behind the seat.
I have a 48 cab that I intended to swap for a 49 on my truck. Well, that didn't work. At least not as a bolt on, no fuss swap. Of course, I'm picking up another 49 cab this week.
