Well I used to own a 67 mustang. There are a bunch of companies that specialize in 64-73 stangs and can get you any sheet metal you need no matter what it is. Some of the companies are: charleston Mustang, Texas Mustang, etc. Pick up a copy of Hemmings Motor News and in the 'Mustang' section will be tons of adds from these stang companies. You can send for their free catalog. I still have about a dozen of these catalogs at home. The prices are fairly cheap on sheetmetal as well.
I always prefer the option of a preformed piece of metal rather than spending countless hours forming my own.