Good suggestions from jhow66 & Cameron.
My suggestion is the HEI unit has a TACH output on the connector. It is the unused terminal.
I guess if you had a stock wiring harness, it would be the unused terminal, but, with a custom set-up, I believe the terminal to use would be the outside upper left terminal looking at the dist cap with it removed and looking directly at terminals, place the four-pin connector at the 12 o'clock position.
Hopefully others here can verify this location.
Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"