I pulled the heads on my '74 Nova 350 today and found these pistons. The motor originally belonged to a '78 truck. I am guessing it was a rebuild that that one of the p/o's put in the car. I am trying to calculate compression ratio (ballpark) in order to get a cam that will work with this motor. I believe that these are dished pistons and I assume the "30" represents a .030 over bore. When using these online compression calculators how much - negative cc's would you guesstimate on these pistons? Also, when measuring piston to deck clearance am I correct that the raised outer ring area of the piston is the area that I need to be measuring from?