I can't figure it out, maybe you can?

I race a 1973 Dodge Dart Sport with a low compression 340. This engine has performed really well over the past ten years but recently I had a problem.
At the end of a run the engine began to stutter and upon inspection in the pits it was revealed that two pushrods had broken through the rocker arm cups.
I took the heads off, checked all of the valves (nothing was bent or damaged) replaced all of the rockers with new ones and put it back together. It's starts right up, idles fine, but won't rev past 5000 rpm (I used to shift at 6200). I replaced the ignition box (mopar orange one) with another orange one but it still won't rev. I put in the Mopar silver box (everyone at the track says its ignition related) yesterday and am going out tomorrow but don't have a lot of faith that is the cure. I've replaced the battery with a new one, the compression on all cylinders is fine.

Any ideas?