D&D Motorsports has a good speedo gear chart I believe. I've heard that for a lot more $$$, you could get the SpeedCal from Dallas Mustang as it is dead accurate.
Basically, there are two types of speedo gears where one is very easy to install and the other requires removing the tranny. Overall, this is pretty accurate. However, if you need the #23 gear, most have agreed that it gets chewed up over time.
Others here probably know the exact parts you need, but that's the basics of it.
Do a search also. I'm interested in 3.55's for mine and found lots of good threads. Hopefully, I can find a local installer who can do the entire job that includes both speedo gears so I can get a pretty accurate speedometer and odometer readings.