ya its definitly do able but you can put a reg cab 4x4 box on a extra cab you just need to do alot of work re doing the rear end to line up properly but I do have a b2200 with the 4x4 box on my reg cab and here is a pic.
I know the box is not level with the cab yet but thats because I still need to body drop the box but here is the B2600i hood the 4x4 fenders and 4x4 box on a B2200