A large part of the stiffness in an aluminum canoe comes from the material being stretched over a form to create a compound curve. This process requires a significant tooling investment - great for 30,000 Grummans per year, not so great for just one. If you did copy a S&G design it would require thicker metal than a strech-form design (.090" aluminum weighs 1.3 lbs per square foot) resulting in a fairly heavy boat. It would also be difficult to weld the seams and end up with a smooth hull. Fixing that would require fairing putty which would also add alot of weight.