MIKE, your duplicating jig is so cool and so essentially simple! Of course, every moving joint would have to be smooth-acting and tight, to hold some semblance of tolerance, thereby minimizing the amount of remaining handwork. Is the one rig large enough to accommodate all the wood members of a Buick wagon?
Up close, those fabricated steel brackets aren't so complicated. Careful layout, cutting, and welding would result in a good facsimile. The necessary accuracy is really achieved by the woodwork. I love the look of the old wood that you've reused in that rear quarter of the '51. That it's darkened and even black in places, is part of the legacy of the car.
The art world and the antique furniture world has understood this for decades and I'm glad the old car hobby is now catching up with that. As long as new material inserted during repairs doesn't "catch the eye", by being completely off-colour, there is no reason to make the result entirely as-new. Unfortunately, the auction industry and its investors (a few of whom are actually hobbyists) stubbornly disagree.
You're living my dream - sleeping quarters above a shop full of fascinating cars is all I ever wanted. And those roofs! Probably because most of us rarely see it, my favourite view of many cars is from directly overhead.