On any closed chamber head, there is a flat surface adjacent to the chamber dish that mates up with a flat surface on the piston crown on a flat-top piston. If you have to increase cc's to lower compression ratio, you would use a dished piston like this that has a D-shaped dish so that you still have a flat spot on the crown to come up against the bottom of the head......
http://kb-silvolite.com/performance....tails&P_id=224