How would you fix this cowl rust?

well there's possibly an alternative...the mustang folks have the same common problem with mustangs, and there's an after market plastic sleeve you can buy that you slide up inside, and it is siliconed into place. It's not the way I would personally fix it, but for those who can't, or do not want to weld in new peices, it's an alternative fix. I don't know if the mustang peice would fit, or could be trimmed to fit but it might be worth looking in to. Maybe call a mustang restoration place and if they have them in stock, see if they would give you the deminsions, and go from there