CP I may have done the thin airfoil and the 25, but you get to be the first to the the thin wood AND the hot 15
As to spars for the foam wings, we did some 1/2a size RC combat planes a while back. Ended up switching to leekie motors as so many of the guys had problems getting 4 surestarts all running at the same time. If you grew up learning bad words by trying to get cox reedies running, it was fine, but teaching old dogs new tricks is tough. Anyway, built the wings by glueing 2 pieces of 6mm foam together and running a hardwood spar embedded 1/2 way through each piece. Also did them with CF spars. Never had one snap even with some hot motors strapped on. The Fox 36 is a different animal. Mine would occasionaly fold a voodoo wing. Never was good at reading the section on "recomended control throws."
I measured one of my current Pacer wings and it is 37" span, 6" in the center and 4 1/2 at the tips. So your 42" and 7 to 5 should handle the 15 easily. You may end up bored and running a 25 with a pipe
So leave room to move the battery back in the fuselage. Come to think of it I have quite a few planes with a 1000 to 1500mah NiCad rx battery all the way in the tail as I tend to opt for the Binford 9000 series motor option. I think that's what you're supposed to do with planes you win. I won an old Rimfire kit as a door prize at a club meeting. The box said use a Veco 19 to 25. I sheathed the whole wing and ran an OS 61 long stroke with a pipe. Was lots of fun for about a year until I had a control surface flutter for about 1/4 second before it self destructed. The price you pay for performance. Little problems become big problems in far less time than it takes to reduce throttle.