Try to get it running if you can. The trans will probably shift OK once the gears move a little. Right now, the synchronizers probably aren't positioned in a place where they'll mesh. That's not uncommon on some of the big rig trannies. It would be very unusual to find a twin-screw rear with 2-speed axles, so I think you'll find that one of them, probably the rear, is a non-driving "tag" axle. Air brakes are no big deal, once you get familiar with their operation. The direct-swap tubeless tire for a 10.00-20 tube type is an 11-22.5. That would require changing wheels, however. The overall height of those two sizes is equal, so there would be no change in speed or ride height.
Good luck!
Jerry
