I don't have the curb weight for the GMC but the '53 Chevy 1-1/2 ton with the 137in wheelbase has a curb weight of 4775 lbs. with a flatbed and 5010 lbs. with a steakbed. You'll need to estimate for the different bed and engine but if might give you a starting point.

http://s299.photobucket.com/albums/mm282/dw_moto/?action=view&current=001.jpg