I put in a same unit in my 37 chevy pickup and I boxed it to the outside edge, then ground the welds smooth. You will find that the cross member will fit tighter this way. If you box on the inside you will have to trim the boxing plates so they will fit flush. It probly tells you in like step 5 or 6 that you will need to cut a C notch in the frame to have clearance for the steering rack, don't worry it's easy just take your time and make sure your measurements are right before you finish welding every thing up.
