What are you thinking for future access (top or bottom), and what type of box? Are you thinking an enclosed (but vented) box or a tray? Your points of support will depend on the stiffness of the box to some degree. One thing to think about is where you're going to run your fuel lines and wiring from front to back. A big solid box in that location is going to eliminate that frame rail as a routing/support point, and your MC mounting plate totally blocks the driver's side rail for front to back support.