I don't think fitting a thicker head gasket will really solve this for the long term, if he fits a new gasket without measuring everything up then it will all have to come back off again if the pistons still hit , u NEED to find out why those pistons hit, and what it will do to the compression ratio.

where did u get all your bits from to rebuild, are they new pistons ??? is the crank standard ? if it was me I would take it apart and measure it all up ( as much as u don't want to I can imagine )