Need a Mopar guru: 273 part-throttle ping driving me crazy!

Float level and jetting are on the right track. Part throttle conditions are very sensitive to air fuel ratio and engine temperature as well. You might also try blocking the heat crossover between the heads and intake manifold. Make sure your coolant mixture is not too rich as well. Too much antifreeze reduces the amount of heat that can be transferred from the cylinders. Many engines also require a relatively high temp thermostat to actually run cooler. The 195 degree t-stat slows the water down so that heat can be more efficiently dissapated in the radiator. The 273 cylinder heads are of closed chamber design and are more sensative to octane as you have found out.