All Chrysler starters have an integral solenoid, since ~1960 and same PN for slant, small-block, big-block until the Magnum engines (1980's thru ~2002). Magnum V-8 use a smaller, lighter starter that fits early engines and works better, so get one if you change. That big contactor on the inner fender is not original, so a kludge. Perhaps it replaces the factory "starter relay" on the firewall. A standard Bosch 30A relay would work fine for that and what later Mopars use. Either way, the NSS should wire to coil- of the relay, shorting it to ground to enable the relay.