I had the same problem and it turned out to be the distributor. I changed from a MP unit to a Mallory which allowed me to adjust the advance better. Problem solved. You might want to think outside the box on this one. I spent a bundle on shroud aluminum radiator fan with clutch and in end the distributor solved the problem.

1963 Chrysler 300 Pace Setter Convertible 21K mile survivor 1969 Dodge Coronet 500 1956 Desoto Hemi 1955 Chrysler C300