How do you wire it into the system on a Superduty ford? I want it to turn off and on just as a 12V cummins should in a dodge.

is this a 12v in a superduty, or do you want to rig up something on a 7.3 or 6.0 im not sure what you want to do here, it seems like you have a superduty in a 12v that you want to make the shutoff silinoid work with your ignition switch

Im wanting to put a 98 12V engine into my superduty. Where do I splice and run the wires to make this shutdown solonoid work properly with the key.

you could do it that way, im not sure where to splice the wires, or you could put the selinoid on its own toggle switch, which would be easier, or even easier you could just put a manual shutdown with a cable you pull

Basically, you turn on the key, and follow the wires down with a test lite.

On the 12v shut off solenoid, one wire is ground, one is hot with ignition and one is hot while cranking. I think you can tap into the s terminal wire for the hold in power, and use a hot with key on for your power to it. Make sure these are on a fuse or breaker for safety.