Now this is a question in my field. I know vintage Mustangs like the back of my hand. OK, a 65, 289, most likely you don't have a 4 Barrel autolite Carb, because they didn't start the GT till 65, and not that many sold. So not having that carb, you loose a good deal of power, which headers are a way of making up for that. Now for the type. The big question is what are you going to do after you put on the headders???? If you decide to go with a whole new exaust system that is built for fryn' rice on a Friday night you want to go with the long pipes. Less of the expensive exaust pipe you will have to use, and easier to get to. Also they have the ability to transfer that loud brute roar of the engine better then just combine glass-packs, and you got a mean import killer. Now if you want that even mellow tone at an idell, and that deep throaty sound that is just a blurr of power, and not something that ripps up like a NASCAR, then go with the shorties. Longer exaust pipe, at a wide guage, makes the sound more of an even tone, and you get a nice calm sound, and a loud, refined muscle tone when you tach up. Now depending on the set up you do you can get the same effect from both styles, like glass packs are always going to give you that loud brute horsepower sound. Note that you MUST re-paint any headers that you buy pre-assembled, because that cheap primer they put on will burn right off. Strip them, and put on high temp. paint, then they will look better longer.
Ives