To replace the whole cowl section is very involved. Several shops told me that unless the car is one of the rare breeds or has great sentimental value to not attempt it. A while back I saw on the web a very detailed step by step procedure on how to make the repair. If you are interested I can try to find it again. (One of the mustang magazines did an artilce on it)
They do make a repair kit which is very inexpensive (less than $20) but will probalby only work on cars with minimal damage. You can find them in most mustang or Cougar catalogs.
In my case I bought another 68 cat that was in good shape but no drivetrain. Now I am in the process of rebuilding her from the ground up adding A/C, power brakes, and power steering.