You would be way ahead to get the threaded rod and backup plate. It doesn't cost much. You will need it for putting this back together anyway. I like that you have it contained, but it's not really a solution since you have to assemble it someday.
Alternatively, OTC makes an excellent tool for this job. OTC7045B.
Check out the angle of the lower control arm with the spring compressed on this SN95. It's not going in without the correct tool in this car.
If you can get an impact wrench on it to rattle it, the lower ball joint will come free after soaking with PB Blaster on the threads. Clean off as much rust from threads as you can first with a rust removal product such as naval jelly. I can't recall if will work on the Firebird, but an extension might fit through the upper hole like it does in the P71.
