George, there are two; one with an external coil (I believe this is the earlier model) and a style that is conventional HEI "coil in cap" design.....either one will work with minimal effort......be aware that there was a California calibration distributor that was used with a knock sensor to retard the timing under detonation (70's era I believe).....that distributor can be changed to a 4 pin module....I just did some testing on a distributor machine and it appeared that the external coil style distributor has a bit more spark output on wide plug gaps; that may indicate that the coil has a more favorable turn ratio but I don't have the specs for either coil.....MSD has a cap adaptor that will allow an external coil to be used with an internal coil cap which gives you more coil options.....(please don't grade the spelling or sentence structure !).....fats #4630
