This is a common problem with MSD ignition and Ford alternators with external voltage regulators and maybe some other arrangements. There is feedback through the charge indicator light to the ignition circuit. Installing a diode in the circuit will remedy it.

The MSD site details the diode installation here

MSD Ignition

If you download this Painless manual for 65-66 Ford the problem and fix is found on page 14

www.painlesswiring.com/Manuals/20120.pdf