.........cut off switch...........

you can wire up the old Mopar relay or a Bosch relay, or you'll have to use the AMC / Jeep solenoid.

One way to do this is just leave your ignition switch / start / start relay wiring all as it is, except

Take the old start wire that went from the original relay to the starter. Run it to fire your Ford relay.

I assume you are still using the switch on the Torqueflite which is the most reliable.

you could of course use a switch on the shifter, and just run the ign switch start wire through it on the road to the Ford relay. Problem with that is if the shifter gets out of adjustment, you could end up starting the car in say, reverse

Several ways to skin this cat