If it were me, I'd just use a hole saw a bit larger in diameter than the upper ball joint threads. Mount the UCA in a vice or fixture so the ball joint is at 0°, drill out the circle containing the ball joint mount, and then tilt the mount until you have the desired caster improvement. Weld the ball joint mount back in.
That way, there's no change in the length of the control arm, so the track width isn't changed any. I know that would be a small amount since you're only extending one leg of the UCA, but it would still change it some.
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After welding the mount back in you could weld in an additional "ring" around the shoulder of the ball joint if you were concerned about the strength.
