How about a stock mustang auto shifter that "ratchets" or at least has locks for manually shifting it? Then you keep all the styling cues of the shifter, especially with the console, but you get all the racey benifits.
Now comes the hard part, the designing it so it will work and actualy making the parts......... *G*. (ask me how I know.......).
It might be easier to take a commercial ratchet shifter and adapt the handle to the mustang unit. Do some market research and find out how many people want one like that, and if you can build it AND make a small profit. Might be a good idea, do some research.
Did the very thing to my 65 coupe, took a B&M shifter and installed it into the stock console without cutting anything, looks real good and works even better. Had to use my loaf/forums/images/icons/tongue.gif a bit but the end result was worth it, send me a pm if you want to know more.