There are two issues with distributors and spark....spark scatter, and spark jumping.....
Spark scatter (as I understand it) is the ability to create a spark at the same time every time during and engine's revolution....apparently the point at which a pick up actually picks up the vane of the distributor rotor can vary slightly.....from different manufacturers.... I know not how the DuraSpark performs here.....but Dr Jacobs of Jacob's Ignitions doesn't seem to have much of a problem with them....
Second thing is spark jumping.... or crossfiring...and this can be reduced by running slightly smaller spark plug gaps than recommended for the large cap systems. Theory being that the larger spark gap works the system harder, and if the conditions are right, a spark could jump from one terminal to another (path of least resistance) and cause issues.
I'd run a small cap and see what happens. If you are the sort that can't handle running below 6000 rpms on a daily basis, well then you need to be more creative in your solution.
I don't know too many folks that actually would ever see their 351C at the 6000-6500 RPM range....so I still maintain that the DuraSpark components are one of the best upgrades for an ignition system.
Doug....what did you say??
Steve