The 16.5s don't have a "bead seat"(not sure of correct name) made into the wheel. The tires can more easily slide inward and come unseated when low on pressure.
USA6x6 and some other companies will make you a double beadlock Hummer Wheel with any offset and lug pattern you want. They cut out the center sections and weld in new ones.
Or just get a set of one tons and use the regular Hummer wheels.