My little brothers truck on 6's....you may remember it from Street Trucks when it was red on 24's......I dont see bodying on 6's.....the tire already touches the hood..........lots and lots of cutting......under the dash brake booster and master.....
If you eliminate the camber in the front of your brothers truck it will save you room to go lower. Tires are taller corner to corner than top to bottom. Also depends on what size tire your running. I believe he is putting a newer front end on the truck that will also give him more room.
if you can body a 03 on 6s then i dont see why you cant do a obs with 03 front clip. the rear should have plenty of room, as long as the front has the new clip, i think its possible.