[Bryan59EC]

I believe you can just hook one of the wires coming from the sensor to ground.

That is how I have the speedometer hooked up in my 59.

Just looked at the instructions for a Rostra Cruise

Hook the gray VSS wire to one of the sensor wires----the other sensor wire will just need to go back to the ground wire going into the cruise box.

EDIT!!!!!!!!!!
Went back and looked again-----your best option would be to use the 2-wire (gray/Black) connector for the add on kit.
Will be doing essentially the same thing as mentioned above ---- Just use those 2 wires (one is spliced to the gray vss and the other goes back to the ground buss in the black box)
and connect them to your current sensor----there is no pos-neg on the sensor so splicing orientation is not important.