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On a scale 1 to 10 ten being hardest its a five..you`ll need some dial indicators, inch lb torque wrench, white grease to check back lash...not as difficult as it sounds but time consuming for the first timer, if your not in a hurry for time constraints go for it, good experience. Get yourself a good manual for the GM 12 bolt truck axle..just keep in mind back lash is critical, if wrong you wipe out the new gears within a hundred miles..
 

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Dont think you can use 308 on a 373 case,unless the 373 is the thick gears.Mine came with 308 and I had to use 373 thick when I changed ratios.If memory serves 373 is a 4 series case,342,308,and I believe 276 fit a 3 series case,A 3 series case going to say 411,456,488 they use to make a ring gear spacer with longer bolts
 

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Been doing a little research on the series 3 vs 4, rear axle casting numbers ending in 140`s are series 3 , casting numbers ending in 170`s are series 4, now these were identified as use in the passenger cars, not sure if it relates to the trucks still researching ... here`s little from Mark William`s GM 12 bolt...

TIP #12 -- CARRIER CONFUSION: When shopping for 12-bolt differential parts, keep this in mind: Chevrolet generally used three different types of differentials for the 12-bolt -- the two series carrier, the three series carrier and the four series carrier. The "two series" has little use in a hi-po application and should be avoided if at all possible. The three series carrier accepted ring and pinions that ranged from 3.08:1 to 3.73:1 while the four series carrier accepted ring and pinions ranging from 3.90:1 to 6.14:1. While there are a number of subtle variations between the three types of carriers, a major difference is the thickness of the gear flange. Two series carriers are the thinnest while three series flanges are thinner than four series flanges. Because of the size of the flange, the carriers cannot be randomly used with ring and pinion sets from the trio of "families".
Unless of course special gears with custom ring gear mounting surfaces are used. These special gears are readily available from Richmond Gear. Additionally, the two series carriers do not have any webs on the top of the carrier. Three and four series carriers do have webs. FYI, virtually all aftermarket spools are engineered for use with 4-series gears, and as far as positraction units are concerned, the preferred choice is obviously a four-series setup.
 

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My understanding is passenger and truck 12 bolts are different.Virtually nothing interchanges.In the case of my 78,from the factory it came with 3.08.Wanting to go to 3.73 because I installed a 700R4,I had to use 3.73 thick gears or go to a 4 series case.These simply call for a thicker ring gear.Passenger car 12 bolts may be different as far as what gears will fit different series cases.Haven't messed with a car 12 bolt in years and it seems my memory is not what it use to be
 

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Thanks for the info. Also, the 3.08 gears should fit in the 3.73 carrier correct?
Nope. 3.08 are 3 series carrier & 3.73 are 4 series carrier. 3.42 is 3 series though, I could be wrong maybe I have the car & truck rear ends mixed up.
 
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