If there is any weakness in your existing ignition system, it's gonna show up under extreme load conditions. Happens on the street, the track and the dyno. I always find a big hill to accelerate a customer's car up if I suspect faulty plug wires, plugs, etc.
Just remember that your ignition system is working exceptionally hard to fire a spark under these extreme cylinder pressures as the engine works hard against a dyno. Modern cars do have superior designs in terms of ignition energy, so the old school Chevys need all the help they can get. As mentioned by Doc, try to eliminate spark scatter with a wide cap, use high quality plug wires and fresh plugs, and make sure you have enough juice to throw the gap for the application you are running. Electricity will follow the path of least resistance, so as it encounters more in the combustion chamber, everything else has to be sealed up tight (so to speak).