Technical Need advice on how to undo & repair a bad roof filler

I bought the car this way and will eventually get to this problem last. Looking for advice on best way to remove and make another. I have ok Fab skills and tool & eguipment. MIG, Tig, plasma, English wheel, shrinker/strecher bead roller, tubing roller. Looking for pics and instructions? Figuring on cutting this out with a cut off wheel and smooth with a flap wheel.

 

When I fill a roof ;
I first find a chunk of wooden strapping,
On 1 side I mark the length, other side width...
Then rest the strapping on the roof insert's outer edge...
With an equal gap at each end, measure how much gap, write it on the ends of the strapping...
Repeat with the width...
.
Take a buddy, the stick and 2 tape measures with you to the parts yard...
Rest the stick on a roof with equal gaps on the ends...
If the arch measurements are close, 1/8" 0r so that roof should fit...
Buddy laughed, till we got to the yard, found out right away what did and didn't fit his '30 tudor …
He found a sunvisorless Volvo wagon... year ?... sorry.
His car... My $0.02.

 

Nothing 15 gallons of Duraglas wouldn't fix.

 

We just watched a HAMBer install a Walden's roof insert in his coupe that looked to have good overlap. Walden hand forms their insert to a wooden form. An option may be to measure your roof insert and see if Walden's will go comfortably outside the welds on the roof to re-establish the roof shape with a new insert.
https://www.waldenspeedshop.com/insetrs-skins

 

Looks just like the '36 I had with a similar quality of workmanship on the roof, mine might have been a bit worse in fact, but mine wasn't chopped. I used a Walden insert which has quite a lot of overlap. I stupidly marked it out and cut to the line. I left a few sections uncut until I was ready to make the final cuts and release the old panel. On making those final cuts at several points there was an almighty bang which turned out to be the badly stressed body (by excessive weld heat) destressing. Funny that the doors fit and closed much better afterwards. Not funny (well for me) was that the excellent Walden insert was now too small (!!) for the enlarged hole requiring numerous small infill pieces - there was as much welding and finishing as two inserts! Thankfully all worked well in the end. A handheld planishing hammer made from a HF benchtop item made the finishing job a lot easier, along with a shrinking disc.

Chris