Yes the MLG was attached to the through spar on the fuselage but the wing panel close to the fuselage requires enough strength to prevent it folding under high g loads. Cutting holes in the lower skin for the MLG to retract into requires the remaining skin (which is in tension in flight) and structure in the area to carry higher loads than they would if there was no MLG cutout. While that is common it does add weight and structural complexity.
Now start cutting into the top surface in the same area and you are introducing additional high compression stress areas and greatly increase the chances of structural failure so require more structure to compensate and more structure means more weight.
Add to that the G forces on a serpentine ammo belt during high g maneuvers would almost certainly cause misfeeds to the gun when the aircraft is turning to the side where the ammo box is not fitted. The belt chutes would not be straight and would require bends to bring the ammo up through the floor and then again to feed into the gun. Both would cause high friction at any time, let alone under high g turns.
Obviously from Shortrounds post they did do this on 20mm armed aircraft and that really surprises me