BioMax
+1y
.08 would work fine, if the frame was going to have a cage attached to it. Otherwise I would steer clear of material that thin.
Unless you plan on towing with the truck, .120 will be fine. Though the problem isn't so much with the material thickness, but the stress at the welded joints. Make sure that you chamfer all of the seams and weld the frame up good and solid. Paying special attention to the areas that will have extra stress placed on them (ie. bag mounts, bed mounts, shock mounts.) If there is any concern, you can always weld diamonds over the welds to reinforce them.