Need Speedo gear for my 64 dart

What you're looking for is a reducer like the one shown toward the bottom of this page:
http://imperialservices.net/Aparts.html

Back in the day they were pretty common on Dodge vans in the wrecking yard..so if you have any wrecking yards around check all of the 62-65 vehicles that might have a low gear ratio no matter what transmission.

You might also try Pat Blais Transmissions....(206) 365-1966