Piston clearance, help needed!

I would imagine KB can send you a replacement piston, so long as you are not in a mad rush... That would be my first suggestion. If it's been mounted (as in pressed on) then scrap the idea of sending it back... Chances are they won't accept it anyway. In that case, I'd just run it. As you say, they do have applications where the pistons are run loose, and I don't think you'll have too much to worry about with .0025 extra clearance there. You have a short piston anyway because of the longer stroke so the ring wear is going to be much faster than a factory displacement.