D4.0 is lots of time to get the merge how you'd like, especially if he is only at 10o'clock. I'd do like Jekyll said, dive into the merge with him at your 12 (turn into him to put him infront of you). 280 mph is a good speed for the Spit, much faster and he'll actually be able to out-turn you (his high speed turn is good). So you should see if you can't fight the battle where he is at the most disadvantage, i.e. slow it down a bit. 9 times in 10 he'll merge and go vertical (either lead turning or long zooming). Watch what he does and respond accordingly. At the top of the vertical is where he'll be slow. The P-51, level and slow, isn't going to out accelerate you or out-turn you in a Spit. Thus, that's the best place to meet him, so zoom up too if you feel you have the speed to convert into alt to meet him on Co-Alt terms at the top.
If he zooms on a hard pull-up, you can probably zoom similar, meet him at the top, and give him 2 options, dive away and play runstang, or die.
-Soda