Yes, standard aftermarket pistons, seem to go to the lowest compression. The factory pistons were "adjusted" for any changes. Original head gaskets were somewhere between .020 and .040. I always milled the heads flat, so you can get a bit more compression that way also. If you mill .020 off your heads, it should put you around 64cc chambers. If you swap for 273 heads and mill them .040 you get down to 57 cc chambers. I would guess casting #302 318 heads would be similar. Where did you find the marine head gaskets at .015 thick? Sounds interesting.