Main armament changes would be:
Reduced cone of fire in the rear tail turret (it would have the original tail, not the Cheyenne tail our -G has), open side windows, equally spaced left/right (nonstaggered), a single gun in the radio room hatch, no "cheek blister" guns that can aim forward (most were angled outward and had narrow windows of fire), only 1 gun in the nose on a flex mount, and a bit of a different performance envelope.
Would still be a problem for A6M2s, I have no doubt. Better than a -G, though!
Also, in WW2 the Luftwaffe learned to make head-on passes on bombers to minimize time inside the defensive gun range. Usually bombers had less guns in the nose because of the bombadier. Later models of B-17s and B-24s carried powered turrets in response to this LW tactic (which proved effective). For scenarios in that time frame before the chin turrets showed up it would be nice to see them over Europe, as well!