You have a good knowledge of the clutch system Andrew, and you performed all the correct measures to get maximum throw to disengage the pp from the disk without preloading the system.
I could write an essay on this, but don't feel like it right now
Since you don't have easy access to the car, I'll just get straight to the point.
The Southbend disk is approx 20 thousands thicker than an ACT disk, PERIOD !