i have pretty much the same engine as yours, 351c-2v . except for mines a 73. But there must be something wrong if you cant go above 75 mph. In my car, all stock, ive gotten up to 120

(nobody has to tell anybody about that one hehe) but ive gotten that fast, and could probably go a bit faster. What transmission does your car have? c-4? FMX? What differential ratio do you have?(curious)