The C6 makes this worthless. It is such a big heavy truck transmission that it will rob all the 460's HP. It may even be faster with the 289 (C4 I assume?) C6 is still a three speed, don't let the higher number fool you. If you have a manual I say don't even trade for a 390 C4, the best automatic ford combo. A manual gives you so much more control, Any mountain driving practically requires it. I can't tell where you're from so I am assuming all different types of roadways. A 289 with any tranny besides a C6 will give you more get up & go than anything with a C6 behind it. It will also give you better mileage than a 460, no matter what that monster is pulling.
289 is also a thousand times easier to find parts for. Almost all the mustangs had it.
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