why wasn't this posted in the warbirds section there is a 30 page thread on this very airframe including engine selection and proping it.
http://www.rcuniverse.com/forum/m_7520206/tm.htm a 15" prop could be used on this plane and clear the ground no problem, so using a 15/9 would work now that being said that size is on the ability limits of your motor, and your likely to bog the engine a bit with that higher pitch prop a 6-4 pitch range would do for the engine if you want a larger blade for looks.
there's areason the 14/8 prop is offered its a perfect size and pitch for that engine size range, and the speed needed for the plane to fly properly.