You can go to the Modern Driveline site, they have a speedometer gear calculator in the resources section.
Good luck with your project.
 

Back in my top loader days as I recall, there were two speedo gears.  one for the trans output shaft and one for the speedo.  You needed both to get the correct readings.  And, there were charts showing what went with what.  Some speedo gears would work with multiple trans gears, but some were incompatible.  I do not know about other transmissions.

My method is very accurate IF you have a working gear now, no matter if it is correct or not.

Use the Speedometer app on you cell phone.  Drive 60mph as indicated by the car speedometer.  Note the actual speed on your cell phone app. 

Doing the math, you can get the percent difference.  Multiply that percent times the number of teeth on your current driven gear.  Then get a gear with that number of teeth.  It works perfectly every time.

The more teeth on your driven gear, the slower your speedo will read.