The Army does not need a folding wing. Thus move also the carb air intake to the inboard of nacelles (also to be used on the Navy aircraft), and there is a better part of the outer wing available for fuel tanks. The P-38 acquired 2x55 gals using just a small part of the outer wing, in front of the main spar.
But then, the fuselage on the historical F7F does not look like devoted to maximizing fuel tankage (my red 'oval'):
actual space on the P-38 was larger than you might think.
30-40% of the volume of the outer wing (not counting ailerons and flaps?
If the 2-stage R-2800s can be installed, the performance would've surpased the historical F7F even in B series engines are used. If not - let's recall that early A-20s were clocked at 349 mph on the low powered R-2600 engines, and F7F was with thinner wing. The turn of speed of in-between A-20 and historical F7F - 380-390 mph - would not be so outrageous to expect.
The
expected speed depends on altitude and time the estimate is made. The
first production contract for the P-61 was placed in Sept of 1941. The P-61s engines were not fitted with water injection (and rated for WER) until the end of 1943 or the beginning of 1944.
Photo of P-61 air inlets:
out board 3 slots (right of the heavy divider) are for the oil cooler. the rest of the outer slots and all the inner opening are for the carb and intercoolers. There was an intercooler on each side of the engine nacelle or partially in the wing/nacelle junction.
Two stage engines cannot be swapped in and out of designs as whim may take the design staff or purchasing agent. If you make the design tight like an F7F you may not have room for the intercoolers and ducting. Take them out of an existing design and you have empty wing/nacelle space (larger airframe than needed. Two stage engines can help considerably with performance at around 15-20,000ft and up. they hurt performance at lower altitudes, also note the 12 ft 2 in propeller. Without a doubt heavier than the 3 blade prop on the F7F.
P & W started work on the "C" series engine in 1940. the first flight of an F7F was in Nov of 1943, 17 months after the first flight of the P-61. At what point was P & W advanced enough in work on the "C" series to offer it as a viable engine to Grumman for the F7F project?
Granted, the proposed changes would've needed to be done when the whole project is in early 'paper' stage.
BTW - the radar operator will have his night vision hampered due to glaring in the radar scopes, unlike what was the case with pilot.
The first gets to the heart of things as to the original question, what was known when design started and what was
desired (like how many hours of endurance and take-off landing speeds).
As to the second, perhaps night vision was impaired a bit but but then F4U and F6F night fighter pilots would have been in real trouble.