I’m with Bob, new shaft is the way to go.
These things take such a hammering, mig weld will most likely be softer than the original shaft and welding the shaft could also weaken it depending on the steel.
If it was something under normal constant load I would say your plan would be fine, but...
It looks like your on the right track from here, 3 terminals will do just fine.
A for the connections it’s simple enough:
First label the end of each winding, U1-U2, V1-V2, W1-W2.
Now connect the wires as follows:
Terminal 1, U1 & W2
terminal 2, U2 & V1
terminal 3, V2 & W1
My guess would be that a 6206 single row ball bearing is rather a small bearing, the two bearings setup the way they are, I don’t think they are up to the axial loading that they will get hitting curbs and pot holes and the likes even from careful driving.
See a 6206 bearing in a mug for scale:
i definitely agree with doing things as big as you can from the start, then you can never look back. But multi tasking is possible, recently I was running a long slow cut in the lathe (circa 45min per pass), the donkey saw was chomping through 120mm bar while i was welding.
Yes the first pic as you say is your main consumer unit and then you have the second one which I can’t make out what it supplies. It looking like there is a little bit going on to start with.
As said how far is the workshop away from your consumer unit?
There are many variables at play and it’s difficult to advise correctly without seeing the installation.
For a workshop with such a large welder and I imagine all the other associated equipment, I would be thinking about splitting the meter tails with Heanly blocks before the house consumer...
The two welders you have listed are 400v 3 phase machines, first of all do you have a 3 phase supply to your property?
Secondly if the maximum your planing to weld is 6mm box these machines seem like overkill, a 3-400a machine is more of a heavy fabrication machine.