The first thing I would do is jack the timing up to at least 12 btc. The factory timing spec was based on real gasoline. Most of us run the timing up until it pings under load then give back 2 degrees.
I have to mention my experience with slant 6 distributers... I've seen more than 1 worn out bad. The point gap/dwell was all over the place. It changed with engine temp too.
Only a reman'd distributer would cure it.