Try these links. Most information out there is Chevy specific. I'm not for sure how the chassis models correlate to the GMC's but they should be similar.
Put big bolt bodies on 4x4 frames is actually pretty common AND totally cool.
If I'm not mistaken, the cabs and grill should be the same, with the exception being the COE trucks. The hoods and fenders are big truck specific.
If you can find a 1/2 ton to 1 ton chassis, it might be a good starting point also.
http://chevy.oldcarmanualproject.com...5559mm441.html
http://chevy.oldcarmanualproject.com...to/53index.htm
http://chevy.oldcarmanualproject.com/
http://www.oldgmctrucks.com/page0005.html