Try cooler spark plugs. Plugs with very deep wells around the center electrode hold a lot of heat that can result in preignition. Cooler plugs have a shallower well and don't hold as much heat in the combustion chamber. I've got a 350 with #16 heads that have been milled to 69cc's. I run 93 octane pump gas and occasionally throw in a bottle of cheap octane booster if I hear a little pinging.

I agree with you that carbon buildup can create hot spots as well. Try the water first - with an atomizer.