jj, we need to know the piston deck height also. That's the distance from the piston crown at top dead center to the flat surface of the block deck. If you don't have a 6" dial caliper, you need to buy one now. The nicest ones I've owned have been made by Starrett, Brown & Sharpe or Mitutoyo. Don't buy any cheapo junk with plastic parts. Sometimes a good value can be found on a used unit on ebay.
Anyway, you'll be able to measure outside diameter, inside diameter and depth with it. It's the depth function we're interested in here. Rotate the crank slowly back and forth through top dead center of any piston you choose (#1 is handy on the front, driver's side) until you find the piston at exact top dead center. Do this by using the slide function on the end of the caliper. Stand the caliper up on end and position the trunk of the tool on the edge of the block, then roll the slide down into the bore at the very edge of the piston right nest to the bore until you get the smallest reading. Standing at the side of the block, measure the piston at either the 3 O'clock or 9 O'clock position. Measuring at the noon or 6 O'clock position will allow the piston to rock on the wrist pin and give you an erroneous reading. You will probably find the piston down in the bore by 0.025" to 0.035" (twenty five thousandths to thirty five thousandths).
What we're doing is trying to determine the head gasket to use. Again, as I've said before, all of this stuff needs to be worked out scientifically. By knowing the piston deck height, we can determine what thickness gasket to use in order to set the squish. You'll need some squish in the motor to get it to run on pump gas with iron heads at 9.89:1 without detonating.