I agree with techinspector. You will save money and parts are easier to find with a 5.0 efi, plus better all-around gas mileage. I would get rid of the c-4 and the gears. You can keep the c-4 if you want to spend the money to get it to handle the 400 hp you speak of, or you can swap in a T56 or a built to the hilt T5. You didn't say what size rearend you have (9" or 8") but it may require some attention before you go drag racing, ie rebuild bigger gears 3.55-4.11 5.0L with boost like rpm to a certain extent, but if you keep a 3speed auto your rpm may be a little high for daily driving with 3.73 and higher. My 5.0 stang runs around 3k rpm @ 70 mph.