The worm gear would have to have teeth that mesh with the bull gear on the main axle. Push comes to shove, you might have to change the bull gear. That would require removing the wheel from the main axle, which would probably require re-quartering all the drivers. Don't know if they still have it, but North West Short Line (NWSL) used to make a re-quartering jig. They may be able to supply the drive parts you will need. You probably should count the teeth on the bull gear before you contact them. Love the boxspox drivers on the Mountain! Pretty good-size drivers. I note that the center drivers are all flanged. That may limit the curves the beast will negotiate.
Best of luck!