RE: Converting Seamaster .40 to Speed 400
Hi Dan,
For your Speed 400 scale-down project, I would use the specs from the old Hobby Lobby Aventura flying boat. This used a direct drive Speed 400 motor with a 5x4 "Zagi" spoon prop on 8-cells.
The Aventura had a 43" wingspan, 34" length, and 333 sq. in. wingarea. The flying weight was around 26-28oz.
Many hop-ups exist for the Speed 400 size planes from longer can motors to brushless to lighter Lithium Polymer cells so don't be afraid if you miss your target. The dynamic range of electric power in this size of plane can easily solve any power or weight issues you run into.