are you now using an internal regulator alternator???
when I got my car, it did the same thing, someone had put an internal regulator alternator into it and did not eliminate the external regulator. if this is the case, the alternator will continue to supply voltage to the coil and distributor even though the ignition has been switched off. the easist way to check for this is to start the engine, turn off the key (so it's still running), and then disconect the T conector at the back of the alternator, that should make the engine shut off. IF it does, you will need to make a decision to either put the external alternator back on it, or remove the external regulator and change the wiring to work with an internal alternator. it's a no brainer to make the change to internal unless you are trying to keep the car completely original. if this solves your problem and you want to stay with an internal regulator alternator, let me know and I explain how to make the changes.