Aluminum conducts heat better, so they're less prone to ping. They usually raise the compression in aluminum heads to compensate for that, actually.
As far as your combo, have you done a plug chop to see if it ACTUALLY running lean? You're kinda guessing and throwing **** at it instead of being systematic. "I'll get the coldest plugs I can get" instead of maybe two steps to see if it's better.
So to be clear, whether you're stopped, or 120 mph, if you go wot, it pings, but if you go 98%, it doesn't?
Have you verified that it actually goes WOT when the pedal is floored? Like, checked to see if the plates are going all the way open? You might be just getting into the mains which are way lean but the carb isn't opening completely.