it is most definatly because you are lean,
i have stock stroke, v3 reeds, very aggressive trail port, t5s, plus 4 timing, stock milled head, 350 mains,30 pilots and have put hundreds of miles on without fouling a plug, and plug color is good not lean.
if jeff from fast or any of the site engine building sponsers suggests a starting point for your jetting you should listen and bet they will get you close.
do not continue to run it with less than 310 mains and bigger pilots or you may burn it up. if damage has not been done now.