I'm surprised you could spin a 21, i would have guessed you would have had a 17 or 19 with the 3.7, we used to run a 21 on a 17 stingray. that being said, i would start with a 17 and see how many rpms you will turn at wot, make sure your tachometer is accurate, you want to see at least 4400 at wot. You may have to go down to a 15
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