Hi Bob, I'm Working on pics.
I,m using a .07x.250 cabon fiber strip for a spar. It seems very strong. I also use a c/f tube as a stiffener on the fuse, (straight down the thrust line). The foam in the plane weigh's 3.5 oz. It has a 36" span.
Did you soak off the paper backing on the foam board?
You should be trying for 3.5 - 4 oz per sq ft wing loading.
Heres a site with some more info, and there is a link to free plans.

http://www.rcgroups.com/forums/showt...&pagenumber=15

Enjoy !
Dave Sloan