You would ONLY move the 2 1/8" front piston assemblies to the to the existing rear calipers, to replace the 1 3/4" piston assemblies. The brake pads (square ones) are the same front and rear. The parking brake attachment is unique to the rear caliper castings only.
The larger pistons SHOULD give more umph to the rears, and result in a more balanced front/rear braking effect with the larger Wilwoods. Suspect that at speed, the Wilwoods under hard pedal would lock up the fronts before the rears with the smaller 1 3/4" pistons. Could result in some squirrelly stops.
Walt
67 2+2