Converted my 66 convertible a few years ago. With the original Rally Pac and using the factory resistor wire for the Pertronix, there's never been the slightest sign of trouble.
Adam, you are correct on the voltage. It drops to approximately half the battery supply voltage that can vary with load and charge rate. So about 6-7v.
1967 Shelby GT350 4-spd #3014 Concours Gold MCA and SAAC
1965 K code Fastback owned since 1977- MCA Concours Gold
1966 6-cyl. A/T Sprint Convertible owned since 1978
1966 C-code A/T Convertible restored 2008. Exercised regularly in season!
1965 C-code A/T Convertible resto completed May 2015
All owner-restored in my garage, never trailered.
MCA #2254 1964 1/2-66 Concours Judge