Snce the block CANNOT come out I'd get some (std bore I'm assuming) KB quench dome pistons of the correct CR & mill them(& the open chambered recess's) to get quench & pull your old pistons & weigh them & hope that they are close to the KB's(if heavier lighten them) & put it together w the block in place! Your piston/bore clearance might be not perfect but this will satisfy your parameters. I'd do it in a second.