Some folks don't seem to know that reach makes the TT length irrelevant. If the reach is the same, then you only need to consider the difference in the STA, that might require a different seatpost setback to get the saddle in the same location, relative to the BB. Stack usually does not include the headset top section, tube and that can vary quite a bit. Sometimes, you can replace the top section with one that is taller or shorter.