OK, here's what I'm finding on the speedo cable situation... going back again to the catalog at Castlemain Rod Shop, they show a cable that SHOULD adapt to a late Mopar clip-on like yours (SC2), or an early screw-on unit (SC1). The SC1 SHOULD also work on aftermarket speedos like Auto Meter as well. I don't think I want to order one from Australia, as there MUST be SOME local shop that can build a cable for me/us.
Another source (more local to me, at least) is Jags That Run / Stealth Conversions. They have a couple options for speedo adapters and Vehicle Speed Sensors. Might be worth a call/email to see if they can source a cable, too, but it might not make a difference if their adapter will let you hook your cable up directly.
