I thought I would give you some kind of a reply on your post.
I have never built the Widgeon, but I have built a G&P Kit. It was the SeaBee. I used the 3M 77 to skin my wings and it worked just fine.
Two old 60's will be more than enough power. You will need to tune both motors to run the same based on the one that has the lower RPM. In other words don't lean the weaker motor to keep up with the stronger motor, richen the stronger motor to run with the weaker motor. I would also suggest a 3 blade prop to give yourself a smaller diameter on the blade. Water spray in the prop is a real pain.

What you should do, is order the video. G&P sells the video for building and flying all of his models. It shows you how to build his plane from start all the way to test flying. Worth the money in my opinion.

tonyc