The 3.55's & a wide ratio Muncie are a great combo with a moderate built 389 or 400 in an early GTO. The factory knew what they were doing with this combo. If you want 3.90's to go with the M21, Richmond Gear makes them & the rearend would have to be setup with them. Unfortunately, the rpms at 60-70 mph with the 3.90's will probably drive you crazy in a daily driver.
If you are considering swapping tranny's, usually M20's are easier to find than M21's & cheaper to boot. If the M21 tranny turns out to be #'s match to your car, I would not sell it. In that case, rearend was swapped out earlier.
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Buzzards gotta eat... same as worms.
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