If someone is on your tail, always turn towards them to give them the worst possible shooting solution.  When I know someone is going to get a shot on me, I like to force them to pull negative g's and turn.  The P38 should be able to outturn a P-51, so I would have forced him into a horizontal turning fight.  The Mustangs stall speed is much higher than the 38s, plus it has all that torque to contend with.  All you really need is a snapshot to put the P-51 out of action.