Well, I started down the path. We'll call it progress.
Initial rough up and flame treat. The backing pieces were secured with plastic weld epoxy initially and then gone over with alternating layers of the g/flex 655 and West 105/207 and glass with sanding in between. If I had to do it over I would have just made some 90 degree brackets with thin metal and epoxied those on added rigidity.
And what it looked like before the finish cuts for the gauges. Between all the layers, the back plates are probably between 1/8" -1/4" thick. Plenty sturdy enough for the gauges.
Now I'm at kind of an impasse about how to secure it to the frame. Originally I had planned on using the factory mounting holes and putting tapped spacers in between the cap and the dash and then securing with a bolt. But, between the cover not fitting perfectly to begin with, and then slightly warping with the flame treat it's off enough that the glove box won't line up. I'm not concerned about the center panel since I was just going to make a flat plate there either way but the whole thing is pretty flimsy. I can heat again and probably get it close but it will involve adding a lot of support to get rid of the flex if I don't address the void.
So, now I'm toying with filling the void between the cap and frame with expanding foam to help with the rigidity. But with that comes issues about making sure the alignment is dead nuts before the pour, making sure the materials are compatible and will bond, etc etc.
Pain in the ass is what this is...