F6F vs a good stick in a Ki84 in a fight below 300mph, you're going to lose. If you have some alt, dive a bit with him chasing you, and make a hard horizontal turn (do NOT immel against an 84), hope he tries to follow you, go vertical and come back down inside his turn for a snapshot. If you get down to low speeds and you don't have an advantage, you better hope he a) can't hit a damn thing, or b) has absolutely no throttle management. Don't follow an 84 vertical, they go up and up and up and up... and up. And then up some more. Your best bet is to give him your 6 at high speed and make him overshoot, since Ki's lose elevator authority at high speeds and as a result, don't bleed E too fast.
F6F vs. a 38 is probably a little more even... you pretty much only need to outfly him, and just avoid going vertical with them.