55 to 64 Chevy cars use the same differential, with the exception of the Chevy II and first gen Chevelle/Malibu. Gear ratios varied depending on donor, including some pickup stuff in that same timeframe. If the guts are scrambled, look for another rearend. Guys have been abusing those for too many years, and repair parts are expensive if not unavailable. And yes, you might get lucky and find a good donor. Best of luck to you.
I put a Chevelle rear end in one... a '69 Mustang 8" in one... a '57 Olds in one... basically anything that is the right width will work. Here's a list of some different makes and their widths... http://www.carnut.com/specs/rear.html Looks like the '55 Chevy is 60" at the flange... (always measure from where the wheel mounts to where the wheel mounts... i.e. flange to flange) So you can always go maybe 2" narrower... but not wider... unless you want to run some goofy offset narrow wheel. Hope this helps! Sam
ford explorer rears are great and cheap. i know not traditional, so i hope i dont get flamed like usual. they are 8.8 like the mustang, but stronger axles, lots of upgrade parts, different gear ratios, etc available, drum brake or disk. came limited slip. can get axles, drums/rotors redrilled to chevy lug pattern. http://www.therangerstation.com/tech_library/Explorer8_8.html
s10 4x4 will fit, cheap and plentiful. gear ratio is often printed in the glove box. most are 3.42, some are 3.73 drum brakes are about the same size too. no need to have two different wheel lug patterns. it only needs a combo u-joint and weld on perches, around $10 (get the mopar performance 3" axle, cheapest)
Late sixties nova and camaros all day, I have one in a 55 and another in a 57 right now and they are other rare 8.2 or better 8.5 because it's easier to find parts for the 8.5. You do have to move the perches just a hair.
Same ones that fit 49-54 Chevy should fit 55-57 Chevy. Which means S10 4x4 and second-gen Camaro 10-bolt are going to be the easiest ones to find that also keep the same bolt pattern.