Alright, so after about 5.5 hours of CAD work, I found that the distance between the top of the motor pulley, and the flat face of the motor (the face that mates with the motor mount plate), ought to be 0.774" (probably 0.775, or 0.78 by design). This number comes from drawing all the parts that go into the pozi drive upgrade / based on real measurements of the actual parts.
Some related distances:
motor pulley top to motor face: 0.774"
motor pulley lower (larger diameter section) top face to motor face: 2.424"
motor pulley bottom face to motor face: 3.984"
spindle pulley top most face to casting underneath spindle side belt guard: 3.807"
spindle pulley underside of larger diameter section to casting underneath spindle side belt guard: 1.663" (the 6 belts distance was 1.762" so its not even close to the same)
high range belt center height to low range belt center height: 1.65"
So, these numbers are nice in that they're made from actual high quality measurements of the relevant parts - not crap measurements made when the parts are installed on a machine. These numbers give the ideal case. There is uncertainty about whether or not my machine's casting is made perfectly, whether the pulleys and belt guards sold to me are made well and designed correctly, etc. So in the end, I will have to set the motor side pulley at the nominal / ideal case number shown by my CAD model, I will then install the spindle side pulley to the ideal numbers too, but then I may need to reheat the spindle side pulley, and adjust it slightly.
I will set the distances using a machined block - as I assume getting the new pulley onto the motor shaft is going to involve heating with torches too. I am about to head down to the chinese tool sellers (HF) and get a hydraulic puller to help remove the existing 6-belts pulley from the motor.
I'll follow up on how all that goes..