You might try to assemble the hull and float it. Could probably mount bulkheads in it temporarily (or permanently if position is safe) and sit in it to find the best position for the seat. It seems 9 ft would be pretty far back for the LCB, just a guess would be 7.5 to 8 ft for a typical hull that length, but 9 ft could be the correct distance for the back of the seat. I'm also just starting to think about a similar design, curious how you are working up the shape of the panels.
Jim