i read on here that you can use a 1986 thru 1993 5.0 mustang automatic transmission  from a donor car to put in my 68 stang and it will hook up . but before i buy one can someone tell me what the polariized  plug on the side of transmision is for? i would to like to know before i buy one i can not use  

The plug on the Driver's side is for the neutral safety switch. I have seen premade adapter wire harnesses to adapt the AOD to a factory neutral safety switch. Also, try to get a 1990 or newer AOD core. Many of the desired upgrades had been made by Ford by then.