Powerflex 306 Front wishbone kit

[powerandtorque]

To change this would require moving the wishbone (or strut top) further forwards or backwards along the car which on a 306 would require some serious modification to either wishbones or subframe.

Not quite - it involves moving the balljoint pin part of the wishbone forward (or the strut top backwards)

If the front bush mounting point is static and you fit an offset bush to the rear (P-bush) mounting point, that'll twist the wishbone slightly around the front mounting point and have the effect of moving the balljoint forwards a smidgen, changing the caster angle.