as above as a general rule standard pistons will protroude slightly from the block where as 'low comp' (i hate that term!) one will fit almost flush.
ALL rst mahles should have some valve cut-outs in them.. if they havent they must of had PURE loped of them!
are you sure they are mahles and not accralites or the like?

Jesse i think standard compression and 30+Psi on Uk fuel would make a machine gun noise come from your engine!

Adam nice 'witness mark' and unlucky about the guide... didnt you turn it over by hand first!?