Don't do what i have seen a number of times and use the feed to your 6v coil as the supply to the new electronic ignition, as suggested in the instructions, as they assume you have a 12v coil. You only get 6-8v.
That said, I saw a SimonBBC unit fitted to a 2.5S saloon, that had, due to the fact it had a much earlier engine fitted, and an electical novice as an owner, both a resistive wire and a ceramic ballast resistor, and teh unit was working (ok-ish) at the 4v it was being supplied.