I haven't seen anyone else comment yet. I'll take a shot at it.
I don't think your going to be able to get that trans to function with the stock speedometer. I don't know of anyone that makes a kit to convert the C4 trans to an electronic speedometer. Though, I have never looked it up either. I just figure the market needs for something like that wouldn't be worth the time to develop.
You might only have an option of fitting an aftermarket speedometer from "Autometer" or something of that nature.
I have a solution for this. You can use one of the abs sensors for a vss. If you are using your abs the just add a bracket and another wheel sensor to one on the wheels and connect it to the vss plug that goes to the transmission. You will need a handheld or a tuner to correct the speedometer as it will be off.