I'm hoping this is not a stock class with set rules? RWYB? or Open, stock appearing? If its an AKRA or NKA sanctioned class then you are limited to 26.5cc and stock piston.
As long as its open, your best bet is a flat top piston and deck the block for perfect "0" then mill the head and keep an eye on quench, leave some room if turning high RPM as these aluminum rods stretch... They also make smaller chamber heads for clones.