I'm with John, I don't think that bushing (or the later roller bearing) is supposed to be so tight to the mainshaft that it will keep the input shaft from tipping. IMO what you are seeing is the accumulated clearance in the big ball bearing that holds the input shaft to the housing. Remember, that little bearing spins pretty fast, and carries a lot of load when in 1st gear. So it can't be super tight. And any movement at the bushing will be amplified at the tip of the shaft, because it's so close to the ball bearing.
IIRC, Bob Schaller (RIP) actually recommended retrofitting a brass bush to the later gearboxes, because of problems with the roller bearing.
.020" is worn, certainly and I would replace it. But not worth trying to re-engineer the joint.
Interesting, if not necessarily relevant: According to the SPC, the factory first tried the roller bearing at TS26825 (with a corresponding change to both input shaft and mainshaft); then went back to the bushing for 11 gearboxes before doing a second change to rollers at TS27063, with yet another input shaft, mainshaft and bearing. Make of that what you will, but my take is that the first change failed in some way, and they had to go back to the old setup while they re-engineered the change.
If you do want to convert to rollers, I think you need to find the later shafts that are machined and hardened with rollers in mind. The rollers are surprisingly picky about the surfaces they run on, and getting it wrong will lead to rapid failure. The input shaft could probably be machined to accept the even later bearings that come with their own outer race, but you would still need an inner race surface, not your existing mainshaft.