I went through this exercise a few years ago and wrote up an article about what I learned. When a wheel cylinder blew back in 2006, I decided that was a good a time as any to upgrade from 9" drums to a slider disc brake system. I used the calculations in the article to figure out my tire and speedometer gear options.
Tire Upgrades
If anyone in Canada needs a P185/80R13 tire, the above article also explains how to get this tire at a reasonable cost.