You don't have to swap harnesses to switch from OBDII to OBDI. The harness for OBDII is the same pin for pin as the OBDI harness, with the addition 4 wires for the rear O2 sensors.
All the sensors are the same, with the exception of the knock sensor which produces a higher value. This can be rectified by either swapping to the older sensor, or wiring a 2K resistor between pin 22 on the blue connector and ground. All you really need is an OBDI PCM.
You can find OBDII programming software now, but it is more expensive (probably by more than it will cost to swap to OBDI) and the monitoring/logging software doesn't work as well. Because of this, I would advise you to swap to OBDI