Hy rt66paul, Welcome, firstly the cabs are the same until you get above the 600 series in a GMC, if the doors roll under at the bottom, then it's the same as a pickup cab. There are two different widths of fenders for the big trucks, 400 series GMCs and larger have the widest. If you're going to use big truck fenders then you'll need a big truck hood as well, they are different at the cowl end, they come to a point there, where the light truck hoods are rounded in this area. I'm cleaning up after a dump of snow but I'll see if I can get you some fender width measurements.
