I'm not sure I can explain it well enough without pictures, but I'll try. I have done this a few times. It is the same proceedure as reversing the old Corvair boxes.

Clamp the box in a vise. Loosen the sector shaft adjusting screw slightly (That's the screw with the locking nut on the side of the box opposite the pittman arm). Unscrew the cap off the big end off the box (I always used a large pair of channel-locks, a pipe wrench will work, too). Carefully screw the worm shaft out through the open end. Hopefully there is enough grease in the box to hold all the recirculating balls in place. Now flip the worm shaft end-for-end and screw it back in place. Everything should look pretty much the same as before except the input end of the worm shaft is sticking out the big end. Next you will need to drill the cap so that the shaft can pass through it and it will need a grease seal. If you can get the original seal out of the other end without damaging it, you might be able to reuse it. Otherwise, you will need a new one. If your local parts house isn't helpful, try a lawnmower shop. The end that has the original hole will need to be plugged with a small freeze-plug so the box will retain its grease. After you screw the cap back on and tighten it, adjust the sector shaft adjusting screw to remove excess play from the box, tighten the lock nut and you're done. The box will now work backwards from before, so you can mount it with the pittman arm hanging down instead of pointing up. Good luck.