WELCOME! WELCOME! WELCOME!
Is this the plane? If so it looks like you should place a 1/8 inch block under the spar at one wing tip with the other wing flat on the board. I don't know for sure maybe someone else here has built one of these. I also think that the 8.5 inches you mentioned must be a misprint.
http://www.checksixplans.net/spit.htm