Bevan,
34" OD is the commercial chassis standard, and that is what mine is. The frame channel width on mine is 3", so 34" - (3"+3") = 28". My spare is 31" and I can just get it to go between the rails by coming up at an angle, putting one side in, and then bringing the other side up and into place. The tire will just sit on the frame rails on each side.