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Discussion Starter #1
My 69, 396 came with a 12 bolt posi 2.56 gears. I have been redoing sections to restore the car and kinda making small improvements but keeping it close to original as possible, the motor and rear gears seems something left to be desired.

I refreshed the engine with a very mild cam and a Holley 4 barrel carb. Older C396 intake, new lifters, and head job. Stock exhaust manifolds, no headers at this time. Dual exhaust, stock TH400 Auto.

What I want to see is if it would be better to change gears in my rear end to a 3 series posi of 3.31 or 3.55 with the set-up I have, or have gearing redone in the transmission to work with the 2.56 gears I have now. I just want a bit more fun off the line. I guess it would be considered on cost since I have read on other blogs that redoing the transmission with lower gearing may be an alternative way to go.

Right now it launches ok, but it feels like it has more but the stock combo gearing is not helping. Given its almost 4000lbs of metal, I know I'm not a contender for the 1/4 mile. Highway driving it considered as well.
 

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Regearing the trans can be $$$$, 2.56 gears were common for factory A/C cars. If you are running large diameter tires then go with a 3.55. Tires closer to stock diameter a 3.31 gear would be a better choice IMO. Do you have a posi rear end? With deeper gears, a non posi will smoke that 1 tire way too easy!
 

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Discussion Starter #3
Broadcast sheet on the car states a 12 boltposi rear end with 2.56 gearing. I have yet to confirm the posi and gear ratio by getting under the car but definitely counted 12 bolts on the rear pumpkin. When we put the motor back in and I tuned it, I did the test burnout and only one tire spun. This makes me wonder if the rear end may have been blown out before I acquired the car and replaced or if it is the orignal gearing that the posi is worn. I'm still dialing the motor in and so far it has nice pick-up.
 

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Discussion Starter #4
Well go under the car and to my disappointment the rear end was swapped out for a 10 bolt. Its an open rear end but unsure of the gears. Doesn't matter, time to look for a 69 12 bolt. I will be selling my 10 bolt though..
 

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For the Rear End Assembly itself, Drum to Drum, all Full Size Chevys from '65 to '70 will interchange (Impala, Caprice, Bel-Air and Biscayne).
 

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Discussion Starter #7
My axle code on the car now is CEA 0504K - 1970 Chervrolet 2.56 non posi 10 bolt. I am thinking of using the rear end and just change out the carrier to a 3.55 or 3.73 posi. Did the 10 bolts on the full sized car in the 70's have 30 spline or 28 spline only?
 
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