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Since the holes aren't very big once cleaned it you could use jb weld . You would probably need to use some tape or something similar for the first pass to keep it from falling through but it would be every bit as good as fiberglass .
You'd have to have to apply it so it was thicker than the metal, especially on the under side. There might also some flex in this area, not much but the cab is always shifting and twisting. If you only apply enough to equal the thickness of the metal of the cab I don't think it's last too long before it would form cracks, at least I wouldn't want to take the chance of happening once the window is installed.. I've been told that POR-15 is a form of epoxy, JB weld is also an epoxy. They both will bond to properly cleaned metal, POR-15 needs a "tooth" to bind to like solid rusty metal or sand blasted metal, it will not adhere properly to sanded metal.
The hardest repairs I've done is around the window of any vehicle. It has to be perfect or you'll have leaks.
I'll second EBEAR. I used JB Weld and brazing to "repair" my wheel arches and tubs. And believe me--what I had was rough:
I purchased replacement parts and was all set to commence with major surgery, but in the end, didn't pull the trigger and opted for a faster cheaper "temporary" fix. I'm four years in with the repair, and so far the brazing and JB Weld are holding up. I always expect to see rust bubbles coming through, but so far so good. Good luck.
I don't think a little build in the area under the rubber is going to affect much and obviously underneath the dash is of no consequence . As suggested the metal needs to be properly prepped and you would likely need to make a few passes to get a base to fill the holes before getting enough to reasonably taper it . Is it the best way ? Not even close . New metal is obviously best probably followed by brass ( some skill involved here ) but I think I would use the jb before fiberglass . JMHO
Here's another pic that shows what some of the areas were like that I filled in with JB Weld:
Innitially, there was just body filler over the brazed repairs and rust bubbles did come through--and in less than a year. I blasted these areas, epoxy primed them, and used JB Weld for filler. It's been holding for four years, but as mentioned, I do have the parts stored for a more permanent (and costly) repair should the need arise.
