[lust4speed]

First, drain the gas and save it for your lawn mower -- replace with premium fuel.

After that, it's just a matter of elimination. Most of the time the problem is aggravated by retarded timing which builds much more heat in the cylinders. This retarded timing also requires a bigger throttle opening, and as mentioned above, greatly contributes to run on.

Since you say the engine ran ok before adding the Pertronix, I would rule out the vacuum leak for now. I'd even make sure that the previous owner didn't swap a plug wire or two when he put things back together.

And last, you might have to check the quality of his installation of the Pertronix. While the unit is fairly foolproof, there are a few clearances that have to be checked. For that matter, borrow a known good Pontiac distributor off someone and give that a try if the easy fixes don't work.