A little bigger is good. You can cut or mill off, but adding to it is not quite the same.
And though it's not something I would not normally do, but a set screw works on many a PTO shaft on the farm that spins well past 1000 RPM and drives many an implement. A dimple in the shaft, some red lock tite and you're good to go. Or even a bolt running through the shaft and yoke with locking elliptical head. I'm not saying these are the right ways to do it, I'm not saying it's better than welding, but it's better than a chicken **** weld. Bear in mind I am about the shafts that are squared off on one side (or both) or splined. Round shafts are another story.
But I remember my old 73 F100 4x4 that had manual steering. The box was so worn out, there was no taking up the slack. But part of that wear was wrestling that huge steering wheel while sitting still on 33" tires. That never helps. I fixed the square worn plastic pieces in the cup that makes up the joint in the shaft, and even tightened up the box a little. But that EXPENSIVE manual steering box was the culprit. I often thought of going with power steering, but love the fact that no one wanted to drive my truck because they couldn't the steering.