1966 Mustang aftermarket design starter wiring

I presume you got rid of the factory starter solenoid then? Factory-type starters do not have a solenoid on the starter like that one...the solenoid is mounted on the fender by the battery. A solenoid on the starter requires all that wiring on the factory solenoid to be transferred to it instead(that lug would likely get the "I" wire if I remember my old solenoid correctly) What you refer to as the starter relay is NOT a relay...its a solenoid and performs the same function as the solenoid mounted on the starter. Assumign I remember correctly, all starter solenoids require at the least, a heavy gauge battery wire, an engine ground(the fender apron mounted one uses a heavy gauge negative cable bolted to the block, but the case itself is sufficient on a starter mounted solenoid), and an ignition wire feeding from the ignition switch.

Are you intending to get rid of the original starter relay? If not, just jumper the big post to the arrowed one one and it will work. That is how Lincoln 460s with Delco starters worked. If you want to get rid of the original relay, there is a lot more wiring to do and you need a longer battery cable that will reach the starter..