Not sure about '70, but it was available as a factory option in '69. Only about 1500 Mustangs total I think, about 250 or so being fastbacks. Main part under the hood, will be in back corner on passenger side and is vacuum operated. The hardest thing to find is the control which is on the end of the turn signal stalk. I found two for my CJ, one used for $150 and a NOS one for $400, lots of internet searching just to find those. Check old Lincolns for parts several Ford models had "speed control" and some of them are not the same for the Mustang. Might have a line on everything but the turn signal, let me know if you are interested. I know someone who took one of a 68 Shelby and might be willing to part with it.