Are the leaf springs for the 54 to 57 Cadillac the same?
54 to 56 probably yes. But there are differences between models. Most probably, 57 rear springs are different because the frame is new for this year.
Sorry, I'm away from home and don't have parts list with me.
It is more complicated than just model. 1957's are all by themselves. In 56 and 57 ,Cadillac used 2 suppliers .Complete springs interchange ,but types A&B have different profiles and ends.
Most 1956 models are on their own. series 75,1954-56. do interchange. 1954 and 1955 generally are the same by model except convertible models,that you need to know serial numbers to get the right one. Models with air ,use heavy duty springs. Cadillac Group# 17.0015
My rule for used springs . Get the pair. From a heavier model in possible.
For what its worth, they can't be considering load and spring rates.
Hollander (1959 book) Rear Spring #11;; 2 1/2 wide x25 1/2 front bushing center to bolt x31 bolt to rear bushing center x 1 1/8 bushings
fits: Cadillac 54-57 , 58-59 Coml
You can sometimes cold work an old spring. Take it all apart. Don't Heat! re-arch the leaves one by one (bend them to create more curve). If they don't break,it will raise the car when put back together. Often short lived. My brother Ed, is still running a 1946 Chevy after 40 years and doing it twice.
Warren