Pony Racer is correct, I found out quite by accident when purchasing the steering stabilizer for my '96 Bronco. The part's guy originally ordered a Rancho universal kit which didn't work due to the boxed (quad shock) frame. The same part number kit that would be used for my F250 was needed for the Bronco. After some research the issue of HD F150's and the like treatment to the Bronco came to light. One of the reasons that the fiberglass cap on the Bronco doesn't self destruct is the frame stiffening by Ford. That is part of what makes the Bronco such a superior off-road vehicle. I have seen several Bronco's and two "HD" F150's with boxed frames. As to upping the load cpaability of an F150, try a set of 5,000 air bags. The non-floating rear axle/spring setup will not allow that much actual load addition, but it is a reasonable way to increase/adjust load carrying capacity. Todays' air suspension systems are a far cry from the old days where they blew out fairly commonly. Many over the road trucks have air-ride suppension today, with very little operational failure.