When measuring the stack height you need to consider the space taken up by the race on the bottom of the headtube as well as the space taken up by the upper race.

To get a better idea of how much stack height your current frame/fork will permit, remove the cups from the frame and insert the fork and then measure the amount of steerer-tube above the headtube.

When installing your new headset you should remove the fork crown race as well and replace with the one that comes with the new headset.

-j