Silver,
There are a number of ways to go about it. There are several factors that have an effect on speedometer accuracy. Tire diameter and rear-end ratios being the most commonly adjusted items. There are two (2) gears that change the speedometer/odometer readings. One in the transmission tailshaft itself and one that the cable attaches to at the tailshaft. Sometimes you can fix the problem by changing the one on the cable alone other times you must change the internal or both. There are also custom shops that will make a conversion gear set for you that resides outside of the transmission. I have contacts for one if you determine you need to go that route. As long as you are using stock z parts and nearly original diameter tires you should be able to find a stock gear combinaton that will work.
You can drive, say 10 miles/kilometers down a road with markers and note the difference between your odometer and the markers (or use a GPS). You can then calculate the percent error. Pull the cable gear out and count the teeth. You will need to change the number of teeth to match your error percent and you hopefully you can find a gear that will closely match. let me know if you need formulas.