What we are saying is this
.. I will be as clear as I can.
You will spend $500.
Any gains in "Quickness" will be so small that they will not matter in the real world and may not even consistently show at the track.
Your stopping power will most likely not be improved because your car set up does not compliment or need rear discs.
So again
.. If you like rear discs, get them. They are cool. But don't buy rear discs if you are doing it for some performance gain because you won't see it.
Your parts issue would be solved by knowing what rear brakes you have.
(You need to know what car 11 1/2" brakes came on)
For example
If those brakes came on a 1972 Plymouth Fury, when you go to the parts store, ask for 72 Fury rear brake parts.
