I was lucky enough to acquire a very rare Eaton Positraction Differential unit (thanks to Jeff @ redneck restorations). These were available in 1960, 1961 and 1962 1/2 Ton Chevy trucks. Late 1955, 1956, 1957, 1958 and 1959 Chevy Trucks share the same rear end and one of these posi units is a DIRECT bolt in for any of the listed trucks if you can find one. The unit was in good condition and was a pretty straight forward swap. I had a 3.90 gear ratio and opted to stick with that because I am going to eventually install a Tremek TKO 600 5-speed which will make it very driveable and very peppy since there is a 454 LS6 in front of it.
All the information you need on the rear end and Eaton Posi unit can be found in the 1960 Chevrolet Truck Shop Manual. This will apply to ALL 2nd series 1/2 Ton Trucks 1955-1962.
I did this more as an experiment because my intention is to keep the truck as stock as possible and did not feel like changing the entire rear end for a 12 Bolt. The original Axle is very beefy 9.3" Ring gear on a 12 bolt carrier (even tho the rear cover is only 10 bolts I assure you it is a beefy 12 bolt inside). The only drawback to these older rear ends is the spider gear do not use thrust washers. Basically what will / can happen is that when the spider gear or house start to "gall" then it is game over for the Differential Case. The gears were not cut with enough clearance to add thrust washers. Well atleast any thrust washers I had on hand. Later model 12 bolt thrust washers fit on the cross shaft but when assembled the gears are too tight and will not function properly. I suppose a machinist could fabricate some very thin thrust washers that could "possibly" work but that is out of my league and this Truck is far from a daily driver so it really is no big deal. The later model rears and the addition of the thrust washer are the main reason they are rock solid axles.
I will add some pics to this thread and hopefully it can be useful for someone in the future.
I recommend to watch this video and all 5 parts of it, I think there is a Part 1, Part 2, Part 3, Part 3.2, Part 4. This is a very similar axle and setting the preload on the carrier and backlash is very informative. The whole video is unbelievable and this guy is a master and knows his stuff. His name is John and it is from Weber State University and they should be credited and commended.
Here is a link to a PDF from ClassicChevy.com that is very similar to the procedurre I did and can be useful as well.
