Assuming the 2 dots on the timing gears were aligned and the engine was not rotated before the timing cover and damper were installed; did you happen to notice if the TDC line on the damper was lined up with the 0 mark on the timing tab on the cover? This sounds very basic but is often overlooked.
With the engine now assembled I'd follow Hank's advice: "I would use a positive piston stop & make sure of your TDC on the damper."
