Based on your information it looks like I need to retract everything I told you! The Corporate Rear used by Chevrolet NEVER had an Electric Shift Motor, they were always Vacuum (at least up to 1967...that's the newest Chevrolet Parts Book I have). When Chevrolet used electric shift it was always pared with the Eaton Rear's.
So, now we know GMC did things different!
I will be of little help from here on out because I don't have any GMC parts book to reference. You'll need to find a late 50's or early 60's GMC Parts Book (which will cover your '57) so you can get the original parts numbers to start your search.
Sorry for confusing things, but it looks like we all just learned something new, which is a good thing!