[webebob]

I'm still choking on the idea that the dyno guys said a mid-rpm stumble was ignition box. Did he say why? Like...was the timing mark flying all over the dampener ...or something?
Did it run fine on the dyno by not on the street?
If that is the case, pull the bowls off the carbs and clean out the crap that washed out of the new fuel lines.
You also didn't mention if you could push through the flat spot.
Either way, I'd make a quick run without the breather and see if the problem changes at all. If it does, it's carb.
I think the swap to MSD is a good move regardless of your problem. But a stock duraspark has powered millions of vehicles to 4700 rpm.
Good luck.