A 350-Turbo is an automatic. It has a Park safety switch. This switch is normally inside the transmission on newer models. If not, you have one of the older models from when I was a hot rodding kid.
I would mount a limit switch somewhere, so that the circuit is opened if the transmission is not in Park. Then route the starter solenoid wiring to this limit switch and from the switch to the solenoid.
Sunday, November 27th, 2011 AT 6:19 AM