Not sure what to tell you. Is it a posi rear? If not then you might as well upgrade to a 12 bolt. On the other hand you said you want speed not torque, most likely the 10 bolt that is probably there now will have a gear ratio that will give you that.
If you find a used 12 bolt on craigslist it may not have the gear ratio you want and may also need a rebuild.
Count the the number of bolts in the rear cover, either 10 or 12 bolt. Lift rear wheels and try to spin one wheel, if one turns and the other turns in the opposite direction, trans in park, then it's an open or non-posi rear. If it's a posi rear you won't be able turn either wheel.
If it's an open rear then block one wheel and put the trans in neutral. Turn the other wheel one full turn and count the number of times the driveshaft turns. That will tell you what gear ratio you have. Could be anywhere from less than 3 to over 4, which I doubt. Be as accurate as you can while counting the driveshaft rotations.
If it's a posi rear end then leave both wheels in the air and put the trans in neutral. Then do the same, spin one tire one full turn, the other should also spin one full turn, and count the driveshaft rotations.
Driveshaft rotations won't be an even number of turns, could be 3.42, 3.73, etc.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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