. . . I am guessing here, but it appears the 63 has the same HORRID cockpit visibility as the 39. That is enough of a disadvantage to counter half of its superior qualities. It will be mince meat for any comparable fighter that can catch one in a co-E fight. . . .
The secret to the " . . . HORRID cockpit visibility . . ." of the P39 is to keep the red guys in front at all times.
The canon is really more of a problem than the thick cockpit braces. Lots of power in the canon but if you are not a dead shot, then you are dead if the other plane doesn't bleed off E quick enough to give you a second shot. The add power of the 63 would help in closure time allowing you get in closer, quicker for a higher percentage shot.
I am interested in the 63 from the standpoint does it live up to the lore real or not. I wonder if it does? Were there dirty politics involved by North American to interfere with an earlier development of the King Cobra? Maybe Bell had lead feet? Maybe the designers could never see the forest for the trees with the less than stellar canon-MG package. As I noted before a friend who flew the 39 in the war had an ashamed tone in his voice when he spoke of having to fly the P39. This stigma may have had deep roots in the pentagon as well.