About a year ago, I found a 3.72 posi 14 bolt off an early 90's 3/4 ton Suburban. It would have been a direct bolt in for the '57 GMC one ton panel that I was working on based on the measurements of the spring perches and the WMS-WMS width. It had eight lug drum brakes that fit the stock 17.5" rims that came on the '57. If you can find one of those Suburbans (or pickups) and want to check the gear ratio and whether or not it has posi, there is a tag inside the glove box that gives the rear end code which can easily be decoded on the interent...much easier than getting out the jacks, crawling under the truck, turning the tires and counting the revolutions!
The rear end on that Suburban had the cable operated ebrakes. The truck I was working on had the drum ebrake assembly on the SM420 tranny, so I was didn't need the cable operated ebrakes, but I made a mental note that the rear end would be a great swap for someone who didn't have the ebrake assembly on the tranny.
Sharing simply as an fyi,
Matt
PS--I ended up using a Dana 60 rear end that was also a direct bolt in (even had the cable ebrakes that I didn't need). It was a 4.10 non-posi. I bought a NOS Dana 3.54 ring and pinion set off eBay.
