I am just finishing my seat on my Ch. 17 kayak and looking at the blue print it shows a Butt Block and Thigh Braces. Also the print states a 112" location to the Butt Block. The instructions are not clear on how to install or where to place the Butt Block and attach the Thigh Brace. Any suggestions. I am a new builder and have learned a great deal about building my kayak thru this forum. Much thanks.