Increasing the distance between the terminals lessens the chance of spark scatter as well as the build-up of ozone inside the cap. MSD offers a full line of Cap-A-Dapts for all Pro-Mags.
MSD Pro-Mags are engineered for a wide variety of racing applications. Each Mag is supplied with a durable cap and rotor that has been tested for performance. When space permits, upgrading to a larger diameter magneto cap can further prevent chances of crossfire. Also, if you're using a timing control to retard the timing, a Cap-A-Dapt is highly recommended because the retard affects the rotor phasing.