This site has copies of the manuals for just about everything.
http://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html
Find the manual for your truck. They have dimensioned drawings that you can print and then experiment with. I've done it for my '57.


'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc