The cab an chassis frames are 4" or so narrower. You will have to redrill the box mounting holes to line up with the frame or build brackets to make it work. The rear axle is narrower aswell (dually). The box bolted to the frame with the wheels centred will leave roughly a 6" gap between the rear of the cab and front of the box. You can buy smc filler panels that match the body lines and fill the gap. Unless its a 10-12' chassis. Fuel tanks also between the frame rails after the rear axle.
There may be more but that's all I can remember for now.