So I did some sleuthing today, as I have both the original 65mm mount as well as an 80mm mount. I put BOTH mounts on the Z-axis rails,
then I measured (as best I could) the difference. I used a woodpeckers square in the 80mm mount bottom, and placed drill bits in the 65mm bottom. The best fit drill bit (I only had imperial) was 19/64".
Finally, I threw all this together in Sketchup and came up with the following:
I would not suggest using this to machine parts, but if you’re looking for the spindle centerline differences in the Y-axis between the 65mm mount and the 80mm mount, this should get you pretty close (19/32", or about 15mm)