OK, Here is how it actually went down.
The Cheata mount did not really fit the stern crossbar and could not riveted in place, so we had to grind it down to fit, and then rivet it on as best we could.
It could not be mounted high enough to get the motor bar level because of the traveler track, which left the motor perilously close to the water, so we taped heavy aluminum angles to the underside of the bar to raise it. This worked so well that I plan to replace the angles with a length of aluminum channel welded to the bar.
Since the aluminum pin that supports the bar is far too weak for ordinary use, raising and lowering the motor as designed is impractical. I fell over the stern finding this out, so I am going to abandon this system and weld a solid block of aluminum in its place. Forget about that pin.
With the bar level, the shroud does not catch any waves in choppy lake water, and with the short shaft, it is easy to tilt the motor up and out of the way when sailing.
Our 3-1/2 HP motor drove the boat directly into a 15 mph wind with the sails up. Very satisfactory performance. It runs about 45 minutes between fill-ups. Warning. I purchased (4) 1 qt fuel bottles from the Bottless website, but he never sent them.
Our biggest difficulty with the boat was being captured in the irons by a strong wind while tacking.
The best feature of the 21 SC is its exceptionally smooth, and powerful ride. We flew a hull with four aboard, and it was like bounding across the waves behind a team of spirited horses.
My Catalina 27 friend said it was 'so damned fast...'
My mentally ill son Richard definitely liked it as well.
Thanks for your input.