NED - The steering column in your 129 is just held in with two short 5/16" NC cap screws, forget if they're 3/4" or 1 inch long. But they hold the gearbox to a cross memeber welded into your tractor's frame. The heads face towards the front of the tractor.
If all your going to replace is the worm slot following stud that can actually be replace with the gearbox in the tractor. It's really cramped under there but working from the under side is much easier. Loosen the jam nut, and the stud should turn out with your fingers.
The mower deck, mule drive, and the frame cover bottom shield have to be removed, and the tractor parked on either ramps, jack stands, or other secure means.
There was a BIG discussion of rebuilding those ROSS gearboxes about a week ago you should look for, also information in the Frequently Asked Questions (FAQ's) you should read. May be more parts to replace once you get into the gearbox.