Steve: I'd use the 1/4" foam for the spar - set CF rods into the top and bottom to really stiffen things up. FG rods can be used too - they're almost as strong and much cheaper - try Balsa Products for those. Or you can use a yardstick like I said back on RCCanada
The foam will wrap around very nicely without having to resort to using a joint at the LE. Make sure you tape the snot out of the outside of the wing skin (run the tape perpendicular to the spar) using clear packing tape before you do the wrap. After the glue sets up the tape can be removed.