Chrysler made two different flathead blocks, one that's 23" long and one that's 25" long. I think the one needed for the '57 is the longer version, but I'm not sure. Either way, they don't interchange, so it might help if you take a quick measurement before you buy anything.
I have a professionally rebuilt starter for a '57 flathead that's 12 volt and never been used. I'll make a smoking deal on it if you are interested.