It's known as the "heat sink" problem. The starter gets too hot, especially with headers, but not always. Ames Performance and Performance Years both offer heat shields for both the solenoid and the starter. They help. Another approach commonly used is to install a Ford starter relay in between the battery and the starter.
Running too much timing (initial past 12) and too low octane can also make it happen. Was this engine modified for lower compression? If not, it REQUIRES 100 octane, not readily available most places.
Sometimes, just installing a heavier battery cable ('0') will fix it. We use high quality welding cable for battery cables in these cars.
Hope this helps.
Jim