I used the Ignitor III because it was thin and I needed that for it to fit in the Accel distributor and it also does multi spark at all RPMs (BTW, the MSD only does that in the lower RPMs). The Ignitor II, depending on the car uses a plastic part that fits over the distributer cam that has eight magnets in it that has been a failure point for some people. The Ignitor III reluctance head fits very close to the distributer cam and triggers directly on the cam high points.
I have fairly long arms so installing the Ignitor III without removing the distributor was pretty easy. Much less effort than changing points. Of course I did all of my mod work on the bench so it was just a bolt in process when I put it in the car. For a stock Pantera distributor it would be an easy plug and play process. You will need to magnetize two of your screw drivers to install the four screws though. It is also a good idea to cover the carb when you are working with small parts so I use a plastic cover that came with the carb.
See you at the Donut Shop tommrrow.