One thing about the 81 harness that I hate is that it still uses an external resistor box. 81 ecu is a good one, no rev limiter.
To convert it, you need distributor from 82-3, and you will need to change the distributor drive shaft, since the distributor base mating part to the oil drive spindle is different. Some minor rewiring is needed to run the internal CAS distributor, but its pretty basic. You also need to get rid of the external resistor box, which can be done pretty easily with the FSM diagrams, the 82-3 models have the resistors inside the ecu. I once ran 81` harness with 83 ecu, and in the long run I fried the harness pretty good. Come to think of it, I would use the 82-3 harness, I dont like cutting that factory harness, and I would source the later harness just for peace of mind.