It's going to be cheaper (as Roland mentioned) to just buy a fiberglass cowl hood and have it painted to match your car... Blending fiberglass to metal is expensive and not very durable. There are many different styles and sizes to fit every form of Mustang ever invented. Just pick up a Mustang magazine from your local store and flip through the advertisements.
Also, when you switch to fiberglass hoods, it's recommended that you start using hood pins as a safety measure. The lighter weight of the fiberglass hood can cause the hood to unlatch at speeds and fly up doing extensive amounts of damage to your car. So do the smart thing and get some pins.
i agree with the guys above. If you were just going to cut a hole in your hood and bolt on a scoop not a cowl then thats easyly done. but in your case i say justy go buy a cowl fiberglass hood.