The XF4U had four wing tanks of 273 gallons, so only room for a single .50 and 10 small bombbbays in the wings,along with flotation bags.
Fuselage had two cowl guns
More armament was desired. More wing guns meant some of the wing tanks and other stuff had to go.
They wanted the tank right at the center of gravity, for the least changes in trim and maneuverability. So it got 237 gallons ahead of the pilot, and 114 in the wings. And a terrible blind spot
You can put a tank behind the pilot, like everyone else did.
The P-51 had one large enough to effect handling when full
Did it need 358 gallons, not counting drop tanks, over Europe or Atlantic?
No.
Late mark Spitfires had 164 gallons, photo recon with guns removed 133 gallons more, Imperial gallons, mind

85 gallons for early.
Me-109 had 88 gallons, Fw-190 110 gallons internal.
Now had they wanted, could have gone with 6 50s( or 4 20mm) in the wings along with 114 gallons of fuel, and 60 gallons behind the pilot and keep the cowl guns and good visibility, and still have more fuel than any other European single engine fighter, since its for FAA and not escorting bombers to Berlin