Karnak, the post-war model (P61E) was renamed the Northrop XF-15 (Reconaissance role)
The four machine guns were installed in a remotely controlled turret on top of the fuselage were deleted on many if not most P-61As, I believe the flight characteristics due to the CoG and "pilot's feel" was basis for decision to remove them, and the fact that the allies had almost total air superiority over the battle fields of both Europe and the Pacific. The P-61 was surprisingly maneuverable for such a large aircraft, but too slow compared to other twin engine counter-parts (P38L) to make a really excellent fighter.
To answer the question? Drop the .50 cal turrets up above(and subsequently lightening it up, as well as enhancing its top end speed) and go with just the 4X20mm.