MSD used to make a standalone unit that provided up to 20 degrees of ignition retard, and could be used with just about any factory or after-market ignition, as it did the retard in the wiring to the ignition coil.
I can't remember the part number right now, but I bought one about 1999 and still have it in storage somewhere.
You might ask MSD tech support what the part number was, then search the web for a used one for sale.