Just posting this as a reference for anyone doing this that my summary above was correct and its all working fine.
The positive wire (this was orange on my AC) goes to the ballast resistor now mounted next to the coil, I added a short length of wire from the resistor to the pole marked "16" on the twin coil to complete the positive circuit
The black wire / earth from the dizzy goes to the terminal marked 1 (this had a plus on it too which is confusing)
The HT leads from the AC set up were long enough to connect directly to the plugs.
I replaced the whole distributer in the end but the normal A/C one would work ok with the twin coil, just remove the rotor, and change the cap for a flat one if you want the complete look.
I have left my old electronic ignition kit off for the moment as its all working fine, but to connect to the twin coil the positive feed needs to go before the ballast resistor, and the condensor can be binned.
The timing was completley out after fitting the new distributer, I just kept rotating it until the engine would fire and run, then set the timing up using a timming lamp. (I bought one with a loop which goes around one of the HT leads to trigger the lamp and its really easy to use)
Changing to a twin coil is actually really simple to do, the coil came from "ricambi-fiat-500-spare-parts-1957-1975" on ebay and the generic ballast resistor came from "accu-spark" also on ebay
Thanks to all for help given and I hope this summary helps annyone else trying this mod
