Welcome to the ranks of the retireded. Are you using points ignition? Because the MSD 5 was used with points only. MSD 6 and newer were for points, HEI, and other aftermarket electronic ignitions. I had an old HAWK/Cal Custom tach in my '68 hard top and I needed the adapter to make it work. Best to download the MSD instruction manual and see what works with what. I do believe someone makes a hood tach that works with HEI and other ignitions, but points is difficult to work with. Most guys put the Pertronix electronic ignition under the stock cap to look original. But besides it being better than points, works better with MSD and modern tachs. MSD book