Holy crap, resurrected a thread that was almost 10 years dormant...
Have the machine shop check and see how much absolutely has to be bored to clean up the cylinders if at all, then decide on your piston size. The amount of displacement you gain from a half millimeter really isn't worth the expense or strength loss, plus finding pistons might be a bit harder, plus you might kill the engine and need to bore on another cleanup- bore now and you'll have less to work with later.