Help me understand my 8-speed uniglide compatibility issue
This is not my area of expertise, but at first glance you have a 6/7 speed freehub body, not an 8 speed as you thought.

I see the threads on the 8 speed are cut into the splines, whereas yours has a continuous thread all the way around, but its lower than the highest part of the splines.

Some photos thanks to google images:





Images from https://www.bikeforums.net/classic-vintage/626072-dura-ace-freehub-id-7400-7402-7403-101-a.html and theres more info there.