That is the standard size of tire for Case GT's and going to a smaller tire just creates problems and does nothing to fix the real problems.
Your tractor is suffering from many years of use, neglect and abuse. If you want it to steer like it did when it left the factory, then you have to correct every problem.
- I bet that at least one side of the front axle is bent rearward from having the tire rammed into objects that caused it to twist. The kingpin holes are supposed to be vertical, not tilted.
- The pivot pin is probably badly worn because no one greased it. A new pin is needed and perhaps the hole in the axle needs to be bored true and bushed back to OEM diameter.
- the top bushing in the steering shaft is probably worn out and you can rattle the steering wheel back and forth.
- The steering quadrant is probably not adjusted correctly with the pinion gear. If you owned the Operator's Manual, then you would have instructions on how to make this adjustment.
- The rod ends on the drag link and tie rod are likely close to be worn out. All four should be replaced.
- If either the drag link or tie rod are bent, then they should be straightened back perfection.
- worn out wheel bearings can affect how well the tractor steers
- if the kingpin holes in the axle are worn, then those holes need to be bored out and have bronze bushings pressed in.
- the kingpin/axle units need to be checked to make sure they are not bent.
- when all the problems have been corrected, then a front-end alignment needs to be done to make sure the quadrant gear is centered and that the toe-in is correct.
These are the things that go wrong with a Case GT after more than 24 years of use. Today, you can expect to pay about $8000.00 to replace your 446 with a new model. So, the question is this. Are you prepared to spend the time and money to truly repair your steering? If not, then you may as well sell the 446 to someone else because it's only going to get harder to steer.
There is no magic fix.