Hi Dave,
You weren't here when I did it, but around a year ago I showed with incontrovertible proof how the Pertronix device fails. The round part which fits over the distributor cam becomes more and more de-magnetized until it finally loses ability to do anything. It is possible that is part of your problem as it is possible portions of that part have begun to lose magnetism. Actually the Pertronix device is just garbage. Sorry...I don't know any other way to describe a product that causes so much trouble and a product that even people who use it take along a spare or a points distributor with them if they leave home. I've also shown on this site how to convert an HEI distributor made for a 230, 250, 292 into an HEI distributor that works fine on the 216, 235 or 261 engine using common tools and an hour or two of time. That's what I have in mine. All GM parts or copies of them and less than $100 will take care of it. The Pertronix unit might make the spark pattern unpredictable.
You asked if a timing advance would help. Do you mean a replacement timing advance vacuum canister or just advancing the timing? The vacuum canister will not help with a low rpm miss. At low rpm the Stovebolt engine relies on centrifugal advance only. It isn't until after around 800 rpm that the vacuum advance kicks in and starts affecting things.
However I still think (as I originally said) that you have an intake valve sticking. A vacuum gauge will show this, but be sure to attach to manifold vacuum and not the vacuum line going to the vacuum advance canister. If the needle drops to a low position and then returns to normal and then repeats the drop in a moment (probably coinciding with the backfire) it is certain you have a sticking valve...but which one? Jerry has told you the best way to single out the valve with a problem (leak-down test). Hopefully you can do that.
