Otto,
I don't recall piston slap coming into the conversation. During my last top-end work I ended up overhoning by at least two thou past book recommendations of "rebore cyclinders and replace pistons." I replaced the rings and all seems pretty good. No particular obvious slapping, so, if slap was the problem on your bike one might have to assume radical overhoning. Personally I don't think that's it. Right now, if it is actually a problem noise and not a usual noise, my bet is on rocker clearance adjustment being accidentally too big. In any case, at least that's easy to check.
Richard L.