1979 Kenworth

The only issue I would have with an aluminum frame in this application is that you need to be very correct in mounting the box and hoist, or the flatbed, to ensure that you don't cause corrosion to occur from the diffferent metals. Put steel against aluminum and if they contact directly you are going to have a lot of electrolyic corrosion, and the aluminum is the loser here since it's the lighter metal. You will need to adequately protect the metals, either with a rubber sheeting or a taping that prevents direct contact.

As for the strength, it's more than suitable as long as you maintain it properly. Remember that the aluminum is more susceptable to corrosion from salt and road goo than a steel frame, make sure that it doesnt have excessive amounts of drill holes in it, and it should be okay.