Erik,
There is not much room in there. The first mod in aluminum is held in place by the angle drive itself. The very first mod I mentioned in MPH required a machined aluminum extension to move the ATD cover outboard for clearance. Peter Sprot's looks like it has a removable face plate, but it would add some weight to the gear. The Ron Barrle mod above is light and attached to the gear, but I am not certain as to his method of attachment.
To me, it seems like a lot of work to do something the Factory never intended. Having ridden all my riding life with the tach provided on my Egli, I cannot remember putting it to good use. Most twins do not get revved very hard even is fast riding. If you are revving your twin hard, a manual ignition with a tach is the best. If you want to use an ATD you will have to make some type of compromise. Peter Sprot's looks like the best, but it is a lot of machining.
On all the racers I run a Scitsu. It would not look right on an old bike, but it is cheap accurate and easy to mount.
David