Internal to external speedo thread adapter?

I'm working on a '39 Zehpyr. The speedo has 3/4 internal threads. I need an adapter to convert it to a standard 3/4 or 5/8 external thread. Does such a thing exist, and where can I find one. Thanks for the help.

 

I had the local speedometer shop build me a cable with an internal thread end to fit an early tach,
they made it look like no big deal.

 

cut the threaded part of a 3/4" fine thread bolt about an inch long, and bore a hole through the center. It worked for me.