I have never built the Huron. These are general ways to do the seats...
1) hang them from the inner gunwales. Typically, they are about 3" below the sheer line. I would hope for at least 3/4" of gunwale to use this approach.
2) screw the to a cleat that is fastened to the inside of the hull at the appropriate height.
~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~Dow's Monaco Project