as far as I know the cabs are the same, even for the bigger trucks. the difference on the bigger trucks is in the fenders forward and maybe a park brake handle sticking through the floor with the 4 spd shifter. the dash is different in the GMC compared to Chev. lots of guys grab a larger farm truck just for the cab because they don't see the use like the smaller trucks did. they are also higher off the ground so they sometimes have less rusty spots. the larger trucks, vikings, will possibly have larger wheel openings in the fenders depending on what series (GVW) they are.
did you find a deal?
check the site below, it will have dimensional pics in there. it will show the different sized trucks and differences in sheet metal but I think you will find the cab dimensions are all the same.
http://chevy.oldcarmanualproject.com...esto/index.htm