This is very similar to the plan I hope to use with my Headliner.
Just a suggestion
I would weld a Jig.
If you run a solid bar with{ tabs (down ward) } where you can bolt to your upper shoulder seat belt bolts.
. Slotted brackets front side or back That would be slotted etc. etc. to hold the roof Up to keep it some what in position.
You could put a Sheetmetal screw to hold it also.
Then use it to support the roof while you are welding/
It will help with the heating and cooling of the roof panel for your plan.
Just suggesting.
Hope this helps.
Your are making Great Progress.
I hope to get back to work on my in a few months.
.