1. Cutting the bottom of the head tube has almost no bearing on geometry. Think of it this way....framesets dont come with headsets yet each headset has a different stack height that could in theory alter the geometry. Seriously.....cutting 1 or 2 mm from the bottom of the head tube changes geometry no more than changing headset or running a 23c tire on the front in conjunction with a 25c on the back. Sometimes its best not to look to deep into things.

2. The 7400 headset has a recessed washer. Use that washer and nothing else.

3. If your going to cut the frame do both ends an equal amount and the amount you cut will be dpendant on the the lug. I wouldnt cut to much of the lug away....cutting both ends will make the head lugs symmetrical.

4.