The P-51 was also a carpet-bomber and not a precision bomber. This means that it's entirely possible that the P-51 would miss the truck entirely. It's infact likely. So let's assume that it missed, because if it didn't this comment'd be over. If the plane was flying, it might be able to strafe the truck. This might also allow someone hypothetically in the bed of the truck to attempt to take down the plane, because we're also assuming the truck has a machine-gun mounted in it. If the plane wasn't flying and needed to take off, I think that the truck would win.