Usually what they did with the NP203 in production is leave the drive gear and driven gear alone and put a Bendix gear reduction unit on it. That was in production, and the service/parts manuals include the parts numbers for those units based on the ratio.
What you need is a solid read on indicated speed at a given known speed. You can get this by running 10 miles on the interstate between mileposts and comparing to the odometer reading difference.
Once you have that ratio, you get the correct ratio adapter.