I'd go for 3.5 or3" BS wheels to help keep some clearance and stability. You'll prolly have to trim the front fenders and tub the firewall a little to eliminate rubbing, or set up some bumpstops to keep the tires outta the steel...
You *should* be ok in the rear, but may have to trim the trailing edge of the fender a little...
It not essential to move the box forward, but it sure is a good idea. The factory mounts were never designed with big tires in mind. I'd plate each side with some 1/4" and box the top, as well as sleeve the holes to make a super sturdy steering box mount. If that fails on the trail...well it just sucks.
Ryan