To my knowledge here are no wagon quarters repopped, except for the '55 Nomad quarters. I replaced both quarters on my '56 Nomad using HT quarters, and had to cut the "dip" out to make them wagon quarters.
First of all, DO NOT make a diagonal cut on the quarter as is often recommended (see pic below). I cut my passenger quarter that way, and it caused a lot more warpage than I expected. On the driver's quarter, I made a 90 degree cut, with a larger radius at the corner. I also cut both sides about 5/8" behind the door jamb. This is because on my car, the quarters were way too long. It may not be a problem on a 57, so you could use the jamb area. However, usually the bend at that location isn't as sharp as the stock quarters.
I think there's a lot less welding on a 57, since the tail fin is split and covered with trim. I ended up leaving my original driver's tailight opening in place, since the new ones were poorly formed on the new quarters.
Here's a pic of how I did mine.
Again, DO NOT do it like this:
Here's some pics of my driver's quarter replacement:
In the first pic, you can see the cut line on what's left of the quarter...
Then I fitted and tacked the new quarter in place...
Finally, I welded and planished the entire weld...
Then I epoxy primed it...
If I were to do it again, I might remove the dip and add the new formed piece to the quarter before fitting it to the car. That way, you can planish and shrink the quarter a lot easier with it off the car.