you'd add ALOT of weight. It WILL affect your payload capacity negatively. Dealer should be able to tell you what teh freight weight of the stock bed is. I know when I did this with my old F-100 as a kid it added 150lbs over what it was with teh steel bed. It was a highway hauler do the increased weight just planted the rear a bit better, and I was a stupid kid and didn't know any better (internet wasn't a thing yet)