[JLMounce]

I want to try and understand this a little bit better with a little more context.

-You have a 455 SD Trans Am that you are restoring. This obviously is a potentially investment level automobile.
-You have the original leaf springs from the car. One side is broken in half, the other side is not, but both sets are likely the born with set of leaf springs
-You want to repair the intact side and replace the broken side?

If having absolutely correct parts on this car weren't a major concern, I would advise against this. While you can re-arch a set of leaf springs without doing anything to the temper, re-arching one side and building a completely new set for the other side introduces some variables that will make it harder to pin down. While re-arching doesn't necessarily effect the temper of the material, it has no effect on the age of that material and any related working stress it has undergone, especially compared to a brand new component.

It's not impossible, but you may find that the spring rate side to side is off enough to cause some weird driving dynamics. You also may introduce some ride height differences as well. It may create some trial and error to get it right if this is the path you're trying to go down.

How important are having the correct part numbers etc on the springs for this car? I'm not even sure if the leaf packs have part numbers struck on them, I'm not that knowledgeable. If this is a critical concern, my first option would be to try and obtain an NOS set, or a complete set that may be used and re-arch those together.

If the springs themselves don't necessarily have to be a factory part, but can be built and labeled to recreate any factory markings, I would have a set of new springs built for the car, then finished, marked and stamped with any identifiers that are needed. What I would be concerned with is making sure that ride height matched the book. This may mean putting the new set of leafs on, measuring ride height at the factory locations, and altering the arch as necessary to produce the correct height. As far as the where to have this done, reach out to guys that may do local circle track stuff. Anybody doing street stock or similar is likely running leaf springs and they probably have those springs custom made. They'd have a good idea on who could produce what you need.

If it is critical that this car have OEM springs on it, my next choice would be to try and locate NOS parts, or even a set of used OEM springs. Again, through installation and measurement, making any determination on whether or not the spring needs to be re-arched.

In the case of above, you could get one spring and rebuild the other.

Having a brand new spring built and rebuilding the old spring would be my personal final option if no other options presented themselves. I would question at that point if you needed that OEM spring, since you would have one that isn't correct anyhow. At which point I'd have two spring packs built to the specs you need.