One thing to report: I did in fact receive my 1" keyed hub in the mail and I did in fact weld it to my splined clutch centre. Did I weld it straight? I sure hope so. I fitted my new 1" keyed hub to the motor shaft and much to my dismay, in very "Princess Auto-esque" fashion it fit quite loosely to the motor shaft. No precision here, folks. So when I tighten the set screw, it pulls the coupler to one side of the shaft so that it wobbles a tiny bit (0.010"-0.020" or so) when the motor is spinning. Hmm...that's not good.
So here I sit on an 11 day holiday at our Hacienda in Osoyoos, trying to think of what I should do. And here's what I have come up with: I will discard the set screw on the motor shaft portion of the coupler to allow the coupler to float on the motor shaft. The clutch splines of the coupler fit quite snug on the transmission input shaft and there is no radial movement of the coupler there, so the coupler's radial position will be located by the transmission input shaft only and the motor shaft will float in the opposite end (the loose end) of the coupler.
There will not be an issue with the coupler moving fore and aft because I welded a washer between the 1" keyed portion and clutch spline portion inside the coupler to prevent it from moving.
The question is, should I put grease between the coupler and motor shaft to prevent the key from wearing out as the coupler moves around on it? I don't know. It will definitely work, but longevity has now become a giant variable.