STA terminal on the alternator was good enough for Ford factory electric chokes........
Do not tap into any power source connected to the ignition coil or distributor, as this may cause misfiring and starting problems.
You obviously do not understand the reason to run off the STA terminal of the alternator.Use a modern relay circuit. You can buy them at any auto parts store for only $6. The get power directly from the battery at 12v and don't rob power from accessorys. I used one to run my electric fuel pump and another for the choke on my street demon carb that requires 12v. search the internet on how to wire one up.