If you are thinking about a heavy over bore, the block should be sonic tested to determine the thickness of the cylnder walls before doing anything. Egge offers the pistons for the 270. My bet is that the ring groves are worn out on your old pistons and the slapping you hear sounds like skirt wear. The std bore on a 270 is 3.781, my guess is this thing won't be able to handle a 4.00 bore, thats a difference of .219!! I would have a Machine shop check the bores and see if it will go .060. Good luck.
Sean
