WWII era Mustangs would have no way to target a modern fighter at all unless the pilot of the modern fighter was acting like a kid playing a video game. All they could do is spray in it's general direction with a hope and a prayer. Mustangs had no targeting except the pilot's visual efforts. Human visual aiming does not work well at the speeds jet operate at most of the time. This was a huge problem by the Korean war.

The F-35 would fly by unnoticed most likely to go hit it's target. If it was noticed the F-35 would not engage the enemy fighters. The Mustang could get to ~40,000ft but wouldn't perform at it's best there, it can't travel anywhere near it's max speed at that altitude and it won't be able to maneuver very well at all. Any modern jet fighter could just sail on by even at 40,000ft without a concern.

Mustangs held about 1800 rounds of ammunition. If they were trying to "fill the sky with lead" they'd expend all their ammunition in about 30 seconds. 72,000 rounds doesn't fill that much sky either.