The 21's got a bum rap because they were about 1oz. heavier than some of the competition. All three, the 19, 35, and 40 run well, but I'm not sure how well the 40 does a stunt run. It has the same bore and stroke as the Lighting Bolt 40, so it should be close.
The Geiseke Nobler was designed around a Fox 35, which weigh about 2 oz. less than the Series 21 40. You'd have to take that into account if you don't want to add another ounce with tail weight. Maybe shorten the nose half an inch, push the stab back half an inch, use a heavier, carbon fiber pushrod.