[puddleglum]

It sounds like an interesting project and I look forward to hearing/seeing how it turns out. I do have one concern about your purposed dimensions. Raising your seating will raise your center of gravity and the rear seat will move the CG back toward the single wheel, although the front mounted engine should help. A narrow trike with a high center of gravity is a recipe for disaster. Thats why most trikes are wide and low. Just make sure you get the design right before you build it. There is some good info out there to guide you if you do some searching. Robert Q. Riley has some good info on this as do some others. Good luck and have fun.