I was about to undertake the same project, but didn’t need to. Your drive gear, with in the tailhousing, should be black. To replace it almost certainly will require an R&R of the tranny. Drain the gear box before pulling the driveshaft/yoke from the output shaft. If you need to replace the drive gear, you can do it with regular tools, but you will need the gasket between the tailhousing and the gear box itself. If you need the driven gear, it can be replaced in the cable without removing the gear box, but that 18 -tooth gear is a bear to find. I think your calculations for the gears are correct, but might check this website...
http://www.boss302.com/speedo.htm
pretty good and well-written.
steve