And if it is mechanical, yes there is a adapter. I bought a whole new cable with the proper ends from a guy on ebay, he makes them up. Tell him your size and the gauge your using and that is it...new cable.
AL
In my custom dash for my 65 wagon, I'm using a 160MPH VDO mechanical speedometer that's actually out of a Porsche. I sent the unit out for a cleaning and rehab (if necessary). The vendor made me a speedometer cable that fits the Porsche unit on one end and a standard Mopar thread on the other.