Paul,

With regards to the piston, .5mm is a fair bit, I would advide that you try and calculate the amount of additional squish you have with this piston and have somone carfully remove the corrisponding amount from the inner surface of the head OR try and find head gaskets that are .25mm thicker than you currently have. I say .25mm as the actual area of the gasket is much more than the area of the piston and .25 should accomadate the additional squish volume.
IF you run the pistons as is with the .5 mm hight increase and nothing else then i would make sure you run super unleaded in it at all times this should prevent detonation.

Good Luck.

BR
Dave.