It's easier to change the oil in the 57. That's about it.
The axle bearings are different so if you need a bearing, you have to order a 57 bearing.

 

I did not realise the 56 rear end did not have a drain plug tried looking in the w/shop manual could not find a clear enough pic, the closest i found was this one

http://www.trifive.com/garage/56 Chevy Assembly Manual/5-5.gif

 

OF....I don't see a drain plug....Is it my eyes or a bad angle for me?

The ring gear in the 55/56 is thinner....So, I would say maybe the 57 is more beefier....(57 on left ande 55/56 on right; both 3.70 GR)

 

OF....I don't see a drain plug....Is it my eyes or a bad angle for me?

Dave,have Sherri check it for you.Its there.:anim_25:

 

case is stronger

if you pull the dropout ( center section ) out and compare with a 55/56 you will see that the 57 has beefier or thicker supports in the housing ( for lack of better words )

 

drain plug

There's a drain plug there, making that a 57 rear end. As was already mentioned bearring size is different. I once had a 55 that had a 57 rear end and I didn't know that till I ordered bearrings for a 55 and they were to small.
Jim

 

Thicker Gears ?

Hey guys isn't ring gear thickness actually based on what the gear ratio is? deeper. Thats why ring gear spacers are used for 4:56 & deepper gears.

 

Hey guys isn't ring gear thickness actually based on what the gear ratio is? deeper. Thats why ring gear spacers are used for 4:56 & deepper gears.

See this post at CT for clarification:http://www.chevytalk.org/fusionbb/showtopic.php?tid/252789/