So the temps out here cooled enough over the weekend, (but the smoke was thick enough to hold off football workouts), that I could spend some time in the garage.
I discovered that - although the brake backing plates are shipped fully assembled - my rear end is Just under the 2.75" minimum axle offset for this kit. (about 2.69"). My installer compensated by adding these washers. (new passenger side backing plate on top of the undamaged original).
(or if it won't rotate, the new plate has the parking brake cable attached already.)
So - Wilwood customer service is going to help me come up with a plan to get this to work. They don't recommend using washers as spacers since washers typically aren't machined to precise tolerances. Could be an installer issue again.
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1965 Pontiac LeMans. M21, 3.73 in a 12 bolt, Kauffman 461.