most definitly lean same thing happened tome in LS my left carb got pluged up and burnt a hole right throught the piston. and all the black marks on the sides of your pistons is alsoa good sign of overheating. have you ran it out of gas or maybe your screans in your tanks are clogged up and it isn't getting enough fuel. since you said you eliminated the air leak issue when you had the cylinders bored did you take them the pistons as well? there is a specific clearence between the piston and cylinder wall and they may be too tight? just a thought. but if i were you i would do a complete tear down replace every gasket and seal, get your cylinders checked, clean your carbs and tank(take the valve off and clean the screans or better yet take them out) and go from there and another thing did you happen to take the flywheel off? if so did you make sure you got it on the woodruff key, if not the flywheel will spin some resulting in timing change.(i have seen this happen before, and believe it or not they will start and run, but not for long)