How do you mate a 4L80E to an NP205 transfer case in a 72 K10, and how do you get VSS?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• LS Control System kit: The brown bulkhead wire is an unscaled vehicle-speed output intended for auto-scaling speedometers. (Chevrolet Performance)
• LS Control System kit: The speed output will not function unless a vehicle speed sensor is connected to the ECM through the harness VSS wire. (Chevrolet Performance)
• LS Control System kit; 4L60 / 4L80: The harness VSS connector must be attached to a variable-reluctance speed sensor typical of most late-model GM automatic transmissions. (Chevrolet Performance)
• LS Control System kit: The ECM is programmed to expect 40 pulses per revolution from the VSS, typical of automatic transmissions. (Chevrolet Performance)
• LS Control System kit; 4L60/4L80 with CP Supermatic Connect and Cruise: The vehicle-speed input is optional, but it must be plugged in when using the CP Supermatic Connect and Cruise transmission control system. (Chevrolet Performance)
• Gen IV 58x (19201327 kit): Attaching a vehicle speed sensor is optional and not required by the ECM in the 19201327 kit. (Chevrolet Performance)
• Gen IV 58x (19201327 kit): A VSS input is not required for engine operation with the 19201327 kit. (Chevrolet Performance)
• Gen IV 58x (19201327 kit): The 19201327 kit's brown wire is an unscaled vehicle-speed output for auto-scaling speedometers. (Chevrolet Performance)
• late-model GM automatics: Late-model GM transmissions used an internal speed sensor outputting 128,000 pulses per mile. (Dakota Digital)

Confirm against the instructions for your exact parts before wiring. More: Tach and speedometer signals in our tuning guide.