IMO, Pertronix III is a piece of crap. My suggestion would be as Mr4X4 and go with a DuraSpark set-up or a Cardone reman or a Cardone Select version of the original Motorcraft with a Pertronix II module and coil. BUT beware, If you want to run 14-16 degrees of initial timing, you may need to modify the limit stop slot to allow only 20* on the crank L10 slot @ .410". This was my experience with my 71 351C 4V with 10:1 pistons on 91 octane pump gas to eliminate spark rattle. All the reman and the new Cardone select I have seen, are set up with 30* on the crank or an L15 slot, which would be right for the factory original 6* initial timing, 36* total mechanical. I welded mine up and recut it. If you get lucky and find an L13 slot, you can add a sleeve to the pin and it will get you close. A piece of 3/16"nylon tube will do the trick, just warm it up so it shapes to the pin. Sorry but from my humble, mega knowledge, it's not Plug-N-Play if you want to keep your ram air (not that I'm lucky enough to have ram air!)
I have posted with pictures on this, but I have no idea how to include the post here.