I made a sort of foolish post some months ago about the HEI system (not
my first error, and probably wont be my last either). The earlier
GM HEI systems had the coil built into the distributor cap, as I remember
Later versions were, I am told, different.
If yours is of this earlier type, you need to be aware that internal arcing
around the coil and even inside the cap itself is something you need to
keep in mind.