The stack height is the height of the lower bearing set, from the bottom of the crown race to where the lower fixed race contacts the head tube, plus that of the upper bearing set, from where the upper fixed race contacts the head tube up to the top of the locknut.
You must also measure the length of the steer tube and of the head tube, and subtract. The difference is the available bearing space. If this number is equal to the stack height, you are golden. If it is slightly, say up to 2 mm less, you are probably ok. If it is a bit less, you may be able to get a spacer to make up the difference from an old headset.
But it really is designed to be equal, and the bearing will be strongest if they are equal.
You also need to double check head tube upper and lower ID, steer tube OD at the crown race, and threading. Swiss frames are less standard than, say, Italian frames.