Ahh, that would explain a few things.
When we got my nieghbors 71 Chevelle, a big tree limb had fallen on the left side of the windshield and conv top. We found a complete windshield frame and header trim and a conv top header. We located them from Desert Valley salvage out of Arazona. Price was reasonable. I had to straighten the left front side rail and the end of the qtr glass rail, and the front 2 bows. Came out great and went up and down fine and the top fit well.
There are several old auto salvages in Colorado also. Come to mind was Rocky Mountain Relics, and Rusty Relics auto salvage. It's a crap shoot if they have a 53-4 top assy. Think there was a couple in Montana too.
I would expect a Pontiac would be the same.