Speedometer cable search

I'm considering buying a "Equus speedometer for my dash insert. I've decided to replace all the gauges. Does anyone have a source for an aftermarket speedo cable. The Equus meter has 5/8 threaded male recptacle, with a 0.104 square shaft input. I think some of the early GM's may have had the same?? (my Dart has a white nylon collar, not compatible). Howard