Rough-idling 360, can't figure it out!

This isn't in an A-body but it is an SBM, the 360 in my '72 D200 pickup is driving me nuts. It has a rough idle issue where it occasionally feels like it's down a couple cylinders, especially in gear (727 auto). I have replaced a lot of parts which needed replacing anyway and/or because I wanted more performance; Edelbrock 1406 on a Performer manifold in place of the stock 2-bbl setup, Pertronix Ignitor 2 kit to replace the stock points, new plugs (just replaced them with .035" gap, had some in with .045" and thought that might be too much but it made no difference), plug wires, cap, rotor. The cylinder heads were also replaced because they had burned/sunken exhaust valves; thought that might have caused the rough idle but again no dice. I did a compression test with the old heads and one cylinder was down pretty low from the others but it turned out to have a badly sunken exhaust valve.

I've tried all the tuning tricks; plenty of initial advance (18*), idle mixture and speed, doesn't change. No vacuum leaks either. I've noticed when the timing light is hooked up and the engine stumbles it looks like the spark drops momentarily. Could it be the original distributor? I'm running out of ideas. Other than the rough idle the truck runs fantastic which is why I'm so confused, as soon as you touch the throttle or you're going down the road it smooths out and feels fine.