My 340 in my 66 developed issues :-(

the ignition system will fail if it's given a full 12v. The ballast reduces the voltage(9v?) while running, but is bypassed for a full 12v during starting.
if you bypass it, the coil runs hot and will fail, and if you are running points, they will **** the bed real quick.
This is with period correct parts.. You bypass it if you upgrade to the various forms of HEI's available