Agree with the others. Need to know the pillar weights as well as weights off the FW sticker. There =should= be a sticker that gives the GVW for the FW as well as the cargo capacity for =that= FW as it left the factory. Do NOT use the "dry weights" of the FW for any calculations as you never travel empty.
Also, I don't know of any GM truck beds that are 5.5' in the Silverado line. 6.5' is the usual short bed length. Depending on the FW, you may, or may not, need a slider.
As MFL said, you need to know the rear axle weight rating (RAWR), but he forgot to mention tires. Likely you only have D rated tires. E rated tires can be used and that will give you a lot more tire capacity.
Post the #'s from the truck and FW here, and we can give you a lot more advice.
Lyle
