The Excalibur was one of the first profiles to use no plywood doublers. The inboard doubler is a duplicate of a inboard plywood doubler, epoxied on, and shaped nicely. The outboard doubler is roughly the same as the ply equivalent, but cutout to receive the fuel tank. Both were 1/2" sheet, but I built mine many years ago ('67/'68).
Some recess the tank into the fuselage plank itself. Be sure to make tank height adjustable, and the tank removeable. Your 3/8 plank seems a bit thin for a .25. Maybe a layer or two of .5 oz CF mat would help, applied with epoxy and not sanded too aggressively. One thing, tho...if you can't mount the engine on aluminum, phenolic, or epoxyboard plates, then mount it directly on maple bearers...not on plywood, even if you use a piece of plywood for the outboard doubler. It squarshes down somethin' turrible...
You might consider building a plywood "shelf" about 3/32 or 1/8 thick by 1" > 1.25" onto the outboard side, under the tank, from the front of the tank, back into the wing. Tie it into the wing, and you'll increase the stiffness a whole bunch. Strap the tank down onto the "shelf", and use plywood shims to adjust height. Consider neoprene 0-rings to sub for rubber bands.
Steve