Voltage Regulator and/or Ballast fails

I have a 1967 Barracuda with a Slant Six. Based on the VIN, I believe the car was originally a V8 small block. I either have an electrical gremlin or something is mismatched in my system as either the Electronic Voltage regulator (2-Pin) or Ballast (2-Pin) will fail occasionally after sitting or when attempting to start. It will work again as long as I continue to replace either one of those parts (depending on which one is broken).

The car has been upgraded with the following:
- Pertronix Ignitor at the distributor (removed points)
- Electronic Voltage regulator (which doesn't appear stock given the year).
- I also assume the alternator has been upgraded to match (how do I know?)

My initial thought is something in the system isn't compatible (alternator or voltage regulator) or something is simply mis-wired completely, but I don't know what to look for given that a portion of the system was upgraded by a predecessor. I'm contemplating ordering a new wiring harness since the old one has seen better days but would rather have a finite answer before attempting to do so, plus I don't know what wiring harness to order since I don't see listings for slant sixes on Classic Industries or Year One's sites.

So the questions I have are as follows:
- What do I check OR is something in my system incompatible?
- Which wiring harness do I go with (original breaker point ignition?)?
- Do I go with a Big Block or Small Block harness (Is one the same as a Slant Six)?