You need to wire the VSS on the tailshaft housing of the trans to the PCM. The Green/Black wire of the VSS should to pin 20 on the C-2 (red)connector of the PCM. The Purple/white wire should go to pin 21 of the C-2 connector. Pin number 50,(dark Green/white) of C-2 should go to your speedometer.

According to LT1swap.com, you must add a pin/wire at pin#38 on the C-2 connector. This is used to run '96-'99 truck fuel gauges. I would imagine that this output signal must go to the location where the old PCM supplied the output signal for the guage to work.