If you want to use the "original" mounting points with a leaf spring you can use a later model (year?) Mopar axle setup and have someone make up a leaf spring setup that has a very short front section (around 19" or so long). The front eye on the leaf can be made to fit the original (control arm) mount point with a little work. The big drawback is that the Mopar axle's spring perches are so wide apart that the inside of the tire is very close to the spring and there is not much room from the outside of the tire to the bead on the wheelwell. An advantage to the Mopar axle is that it has the same lug bolt pattern (4 1/2") as the Tempest does. Another advantage is that the 57 Pontiac does not have posi rearend parts whereas the Mopars have posi units.
I have a parts car that had one of these Mopar rearend setups with the "custom" leaf spring setup. The guy I bought the car from ran the setup with a 455 turbo 400 setup for awhile and the problem he had was the differential hitting the floorboard under heavy accelerating. He didn't have one of those Mopar snubber block setups on it. He had the "custom" leaf spring made up by his blacksmith uncle who shortened the spring and rolled a new eye on it. He had a 3.08 or so ratio posi unit in it.
There are several Ford and Mopar rearends that have nearly the same overall width as the 57 Pontiac. I think that maybe the early Roadrunners/GTX type cars had a rear with spring perch spacings the same as the 57 Pontiac. Same thing for some early Mustangs too as I recall.
Bottom line is that you can make up housings with perch spacings as you wish so that isn't a big deal. I would however point out again that most of the Ford and Mopar rears have the lug bolt pattern interchangeability with the Tempests.
You can see some pics of a leaf spring setup I am building in the thread listed above by c5farmer.
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