I really like the simplicity of the 1990 Regal rear strut modification and the comments from Monroe regarding the dampening being the same as the Reatta 71966. This, combined with the add on sway bar clamp clevis that Daves89 has found make this a great solution.
(Just thinking out loud now...) The clamp on sway bar clevis also makes the idea of a 'from scratch' insert strut holder very possible as nobody has offered up cores for modification. Tube cut to length, weld a cap on the end, thread the other end, add two dimples for the pinch bolt (two so the strut insert is centered). If using a Koni adjustable inserts, the rod end thread length is correct and the strut could be tuned to your liking.
For now I am going to shelve the concept of the gland nut weld adapter and the sleeve nut. I can always resurrect it if needed. I would suggest for those in need of new struts, go out and get a set of the rear 1990 Buick Regal struts, take the flanges off and add the dimple and buy the clamp on sway bar adapters.
It is wonderful to see how this group has come together to find various possible solutions to keep these great vehicles on the road.