If it's a factory 1970 6 pack motor, your pistons should be about .02 in the hole. If it's not a factory 6 pack motor from 1970, then your pistons should be about .09 in the hole. That is assuming your block's deck, pistons, rods and crank are all exactly to blueprint spec.

906's with a .006 shave are nowhere near 78cc. Stock open chamber heads are closer to 90cc and you can start working backwards from there based on how much you shaved off. I forget the formula that shows how much to shave off heads to get a 1cc reduction. Stock steel shim head gasket thickness is around .022, but that's no guarantie that's what you are running now.