If a new plug doesnt get a better idle, then I'd suspect the idle mixture is too rich. You may try leaning the idle mixture before switching plugs. Either way, once the engine will idle reliably, re-set your main needle to peak performance and slightly richen, then re-set your idle needle so it produces a nice crisp transition to WOT. I usually set my smallblock carburetors to stall out at about 3 seconds when the fuel line is pinched.