Elysian,
I think my training site would be very helpful to you. There are many films there of 1v1s that demonstrate great tactics on the merge. The link is below in my signature. I suggest you take a look at the 109-G10 films - there are a couple there that show exactly how you should merge with a P-51D.
But I'll give you some ideas here as well. First of all in like versus like. I am a very aggressive pilot by nature. I go for angles any chance I have. So on a merge with a like plane, I am going to immelman hard, dont be affraid to black out. I will chop throttle (possibly kill engine) to bleed speed and attempt to gain angles quicker by getting slower faster than the enemy. Granted this isnt a good idea if you are going 150mph, but assuming you merge somewhere between 250-325mph, this is my tactic. Immediatley as I merge, I am going to watch the nme plane to see what he does. I do this just before I pull hard into my reversal. This helps me figure out where I need to be on my reversal to get angles. As soon as yuo get behind the nme on the first merge, its all downhill from there. Stay aggressive, but dont be afraid to chop throttle to stay saddled up.
Now, in a 109-G10 versus say an F6F ... well your tactics are entirely different. In a merge of these 2 planes, the G10 should not pull hard on the merge. Instead, it should do a gradual climb, and immelman up over the top with lpenty of E. The slower plane will not have a chance in hell to follow you up. When you get above the nme, start to pick your passes. Always try to approach the bandit from the rear hemisphere, or setup an attack so that at least your guns solution will be from the rear hemisphere. As soon as you determine that you will not have a shot, break off and extend/climb. Be CAREFUL not to extend/climb AFTER you overshoot, as this will generally give a snapshot to the nme. Patience is the key in this fight. But since I am not patient

I would try to get the F6F slow and under me ... then I would chop throttle (possibly kill engine) and roll down to get angles. The G10 films I referenced above are GREAT films to see exactly what I am talking about. I kill my engine both times so that I can gain angles quickly and avoid the overshoot.
I hope this helps some, go check out those films! If you wanna get together for some 1v1 in the TA soemtime, drop me an email,
nimitz@huckabay.netS!
Nim