There have been a couple of threads just talking about this.

The first thing you need is to pick up your electronic speedo. Try to find out what type of signal pulse it requires.

You also need to find out if it needs a 2 wire or 3 wire sensor. More recent speedo's need the 2 wire one, but check to make sure.

Once you have this information, you need to change out your cable out of the transmission with a Vehicle Speed Sensor or VSS for short. They likely have one at the place where you are getting your speedo, but if not check out painless performance and do a search for Vehicle Speed Sensor. They have 3 to pick from and very good customer service - but many places stock them.

Then it's just a matter of running the wire between the speedo and the tranny and calibrating the speedo. It's not difficult and resolves some of the "Bouncing needle" situations that many experience with cable drives.
Jerome