When a nut is rounded, there’s only a few ways to safely loosen it without damaging the thread.
Here’s a way that involves filing down 2 sides of the nut, so you have 2 flat edges, then with a close fit spanner and a wedged in screw driver, you can then have the leverage needed to open it.
I eliminated some of the slop in the monkey motion shifter on my Datsun 521 A type transmission. I used a stainless bushing from an old “Body By Jake” workout machine. It sounds weird but it worked. A stainless 9/16″ (I think) bolt fit inside perfectly while still retaining the OEM spring. Lock washers and some high strength Loctite should keep it all together.
It’s not perfect but much better than before.
The movement in neutral before was roughly 7″-8″. Now, side to side movement in neutral is 3 1/4″. Shifts in and out of gears, including reverse, are slightly more stiff and much more crisp.