A pair of 1/4" beech or ramin dowels anchored in the plywood bulkhead will take everything that plane can give in the air (provided you don't add too much weight), and when the balsa meets the pavement it's often better to have wing hold-downs that 'crumple' to reduce damage. Your hold-down blocks look like you could do chin-ups on them. If you dig a wing tip in a hard landing, either the wing bolts will need to shear quickly or the wing will snap along it's length (or pull a section of fuselage out).

Old Russian saying: "Perfection is the enemy of good enough."

The LT-40 twin has a pair of .25s in nacelles.

I'm guessing you'll want the wheel axles at the leading edge instead of 1" ahead.