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Fighter aircraft powered by the massive R2800 engine were in development pipeline during 1939.
Building lightweight and inexpensive P-36 for sale to friendly nations such as Australia, Netherlands, China, Finland, Belgium etc. might be a good idea. However U.S. Army Air Corps would not settle for anything less then an over priced monstrosity such as P-47. It's the American way.
Apples and oranges.
P-36 is an Army land based fighter aircraft. Historically replaced by P-40.
F4F is a CV based fighter aircraft. Historically replaced by F6F.
.....USMC land based F4Fs muddy the distinction a bit. However if given a choice (and similar engines) the Jar Heads might prefer P-36 over F4F when operating from Guadalcanal.