I think it would be extremely difficult to assemble that the way the plan shows. The more normal way to assemble a foam wing is to place the lite ply spars into the foam cores before sheeting the wing. Since the foam cores are sheeted while still in two halves I can't imagine having all that hardware in place when sheeting the inner half and placing it back in the foam shells for curing.
The more normal way to do it is to install the lite ply spars and landing gear "clips" into the foam then sheet the foam cores and place them back into the shells and weight them down for curing. After the cores are sheeted a notch is cut along the inner spar both top and bottom and the bellcrank pivot rod is inserted into the notch and epoxied onto the spar with the rod extending on both sides of the wing with the flap pushrod in place after the necessary clearances for it are cut into both wing halves. The wing is then glued together at the center while in the outer shell bottoms and weighted down into the bottoms for curing to insure good alignment of the two halves. After the wing is installed into the fuselage, plywood pieces are installed on the top and bottom of the wing against the fuselage sides. These plywood pieces have holes drilled to capture the ends of the bellcrank rod and are epoxied to the wing, fuselage, and bellcrank pivot rod. Then the entire center section inside the fuselage is covered with 2 oz fiberglass cloth and epoxy.
Another safety feature that I incorporate is to dremel the ends of the pivot rod flush with the plywood supports and epoxy a plywood cap over the ends of the pivot rod to prevent any possibility of the pivot rod moving vertically. This of course is done prior to covering the center section with fiberglass cloth.
I would also mention that if you were to buy sheeted foam cores from Bob Hunt or another supplier it would definitely be necessary to follow the above procedure (or a similar one) because they would be already sheeted with the spars installed.
Randy Cuberly