Actually the 69 383Hp engines had a Positive deck. In other words the piston actually was above the block deck surface. I do not have my book in here but I believe the 67 440HP Pistons with the 915 closed chamber heads were at a lower deck than the 1969 and 70 440's with the open chamber heads. I am sure someone will chime in with the numbers. The deck differences is what kept the compression up on the 68/69/70 engines with the open chambered heads. if you punch the numbers into a Compression Calculator (Bore,Stroke, Head volume CC, +/- Effective Dome CC, Deck Clearance, Compressed Gasket thickness, number of cylinders). You will see the factory numbers are real close.