I'd suggest that you go the less expensive route, and build up the spirals with hardfacing rod. If you do a smooth job, a minimum of grinding will be required.
I would do it right and go the change-over kit to the new style, the extra 1200 dollars will be worth it come trade-in or resell, and also why spend time greasing those darn bushings every couple hours, your time is worth something when harvesting.
i just purchased a re-built 963 head. the previous owner installed the kit you mentioned..... the bushings in the slip clutches are rather worn and appear to have a little movement or "play" on the hex shaft.....when i replace the bushings....is there still a little movement to be expected or should the slip clutch have a tight fit on both endsIJIJ