welding on main shaft?

The shaft is worth core money at the very least.

They need to be spray welded, and then ground with a tool post grinder. You may get a decent finish with a tool on a lathe, if you could turn it fast enough, had it rigid enough and had the correct tooling.

Again, you are making something from nothing. I used to turn them down and put a bushing in the input. Sometimes I even use the bushing in the input if the trans is going to get beat on.

.005-.008 isn't deep enough to make me bother. It will run like it is. If nothing else, sounds like you have a lathe. Make the bushing for the input. Lightly polish the shaft and go.

Just remember to drill the oil feed holes in to the bushing.