Er, your math is a little off -- there was no Mustang fifty years ago. The Mustang came out in 1964, and its generation lasted until 1973 (citation).
I had a '69 with a 350CI Cleveland. That small body and huge engine made the button completely unnecessary. To do a u-turn from a stop you just cranked the wheel around, gave it a little gas and popped the clutch, it would spin around with a screech and a cloud of smoke. In fact, the tires would screech every time you power-shifted it, in all four gears.
I really don't know how fast it would go, the speedometer only went to 140 and I buried it once. It was still accelerating when it felt like it was going to become airborne so I slowed back down to a less suicidal speed.
Great engine. Too bad all the things that hung on to the engine, like the starter, fuel pump, etc. were crap. I spent more time under he hood than I did driving it. I think I went through 5 starters on that lemon.