I didn't like my advice and re wrote it. Sorry buddy. Didn't mean to mess up the thread.
We have had others with this rust area. This is THE most dangerous and difficult fix you could have on that truck. The welding is not a problem. Welding is welding. You look like you are doing a good job. The problem with the TF trucks is the wraparound windshield. The shape. The sort of compound curves. That windshield is one of the most difficult to install. IF the opening is built by the factory to one side of the tolerance zone and the Chinese glass is built to the other side, it's a nightmare to get a fit that seals.
Cutting out pieces and fitting them is a little risky. The weld gap you leave/the fit-up you do is hard to measure. Hard to be sure you have maintained the opening to spec. Some days for this repair, I use the rule of thumb: Only repair the rusted area, don't cut anymore than is necessary. That's fine for all the rest of the truck. But some days in order to maintain factory shape, the pinch weld and windshield rubber fit, I think it's way better to install the whole window frame or whole top.
I'm sure you have ideas. I think the straight sections, bottom and top, would not affect the opening size. Except possibly the "Y" axis/Height. Easy to measure. The problem is replacing areas at the curves. One method is a template with registration marks in either side of the cut. Marks on the truck opening in places not being cut; and marks on the template that match. Use the template, with marks aligned, while tacking the patch. The cardboard template should lay ON the pinch weld and should match the opening curvature not the pinch weld, I'm thinking. If you have cut the truck already you can use the donor for the template. You must take all the measurements and templates you can before you cut.
You need a second donor to go back and measure as you patch.
As said, temporary support is need if you think your cut will cause movement/sag/etc.
Any area that does not affect the glass opening, would, of course, be a normal patch made from the donor or vendor.
