Gerrit: The cockpit footwell is same size as the plans; about 20 inches have been added aft of the original transom to accommodate the motor.
Does this mean you extended the length of the boat by 20 inches? It is now a Bellhaven 21? If so, hopefully you had plans or approval for doing that as the additional length would disrupt a lot of design parameters, such as center of lateral resistance, ballast location, etc.
As I recall, the usual plan is to have the motor mounted on the transom. My Montgomery 17 has the same arrangement and you do have to watch how close the motor is mounted to the rudder, or in a hard turn, the rudder will get into the prop. If that is the case, you may want to put a lashing on the aft end of the tiller to limit its range of motion. When under power, because the tiller is running in smooth laminar flow adjacent to the prop, I have never experienced the any tiller vibration under power. Would be curious to know what caused that for Scott.
The Princess 26 plans have the motor mounted directly in front of the rudder, such that when under power, rudder will be operating in the prop wash......at least the upper portion of it near the surface. Bottom half will be in smooth flow. That seems to work well on the Princess 22 boats. On Wayne's Princess 26, I seem to recall he did something similar to the setup above, moving the motor slightly to starboard in a well, such that the motor could pivot on it's own mount, vs. riding a vertical sliding car. He also put a slot in his transom. We know he has motored some distance with it, but have not heard how it is working for him.