He's prob talking about the "I" terminal on the 66, the new mini prob doesn't have it.
The "I" terminal on the starter goes to the coil and provides a full 12v when the engine is cranking. After it starts the "I" terminal goes dead and the coil recieves 9ish volts via a resistance wire. A constant 12v to the coil will burn it up.
Modern HEI systems do not require the "I" terminal because it uses 12v all the time.
You can wire in a simple relay to provide the missing "I" terminal voltage or switch to an HEI system. I'll draw up the wire dia if you need it.