Custom Pertronix question

Rotor phasing is absolutely an issue, and with the poor and expensive availability of parts for those old girls, it might be tough, and you'd have to cut up a cap in order to resolve it. If you are enough of a craftsman to make the new mount variable (to change the trigger timing) it could be done. I would not. Before I did Pertronix (because of their proprietary rotor/ trigger wheel) I would machine someone's reluctor whether Ford, Mopar or HEI, and adapt it to a magnetic trigger or even Hall effect. And with that you could run a choice of some different modules

What you need, is a way to "justify" the rotor position at the point of trigger. If you don't want to cut up caps, If you were able to construct a test gap that accurately reflects a tower position on the cap "exactly" you could also do it that way.