[dsraven]

if you do anything BUT slide the body it will cause your wallet to get lighter by the same amount as a new driveshaft.

if you slide the box back you will have to fiddle with the running board length.

if you slide the front fender back to keep the running board length correct then the grill won't fit proper against the front face of the fenders. not that I know exactly how that looks because I haven't worked on that year, just assuming by checking a pic of that model online. would it also affect the hood fitment?

can you simply slide everything back on the frame, including the cab? figure out how much you need, oval the frame holes by that much where the body mounts go through, place a flat plate larger than the mounts (with a hole drilled to match the body mount bolt hole diameter) on top of the frame to body mounting pad, so it will become the "new" contact between the frame and the body mount, get the body where you want it, bolt it all down and ensure it is square etc, then tack the plate to the frame mount, remove the body or jack it up enough to weld the "washer" onto the frame mount. then put it all back together. done.

what about just moving the rear axle spring perches ahead by the amount you need. you will need a new driveshaft length probably. but it may be easier than all that other messing around and that way all your work so far, to get it right and straight, is not lost.

you could also get a spring shop to fab you a new main leaf for the rear springs. one with the center hole further forward, then replace the main leaf and center bolt in the spring packs. also a good time to replace the bushings in the ends of the spring, not absolutely required though. that may be cheaper than doing the offset mount adapter and a also quicker turn around if you have to wait for shipping on the adapter. this would also require a shorter driveshaft.