Q: Ford Solenoid Switch in Lieu of Mopar Starter Relay?

Ditto. A standard 30A "automotive relay" can replace the Mopar starter relay fine. Wire coil- (pin 85 in post #8 ) to the NSS (grounds it in P or N). That is exactly how my 1996 Voyager starter circuit works, and I don't think its engine computer interacts w/ the starter circuit at all. As 67Dart273 did, I put a newer underhood fuse/relay box in my 64 & 65 A's. I used a box from a 95-99 Jeep. See my post "Modernized Engine Wiring .." (or such) for details.