I have a F150 Super Cab 4x4 with Off Road & Towing package and a 5.4L. I was wondering if it's possible to modify to a 7700 GVWR or at least up the payload.

 

I've only heard of two companies that can legally alter a vehicle's GVWR. Nothing you do as an individual can change that rating.

As for the parts themselves, there are a lot of differences between the regular F150 and the 7700# pkg truck. Frame, axles, springs and wheels come to mind.

To improve the handling of your truck while hauling heavy loads, I recommend looking into air bags or 'helper' springs.

 

There are way too many factors that go into the GVWR, radiator(s), engine, trans, gearing, axles, springs, brakes, tires, rims, wheelbase, shocks, etc etc.