Any ideas as to why the driver's side is 4-5" too low?

 

Still nothing.

Need to replace calipers first anyway. Right brake is smoking because it's always engaged (constant right steering) and when I do brake, I veer left because the driver's side grips much harder, much faster.

More fun to be had.

 

Question 1
Your list looks correct. At least all the cars I can think of. It should fit those cars, but QC wasn't the best and I've found parts from one nova to the next fit a little different.

Question 2
No idea how to test springs to see if they are bad.

Question 3
Yes, you can adjust the height of the rear with leaf springs. Here is a good thread to read. I had the same experience as Philip and ended up using the same springs from Summit. Part # SUM-7081GM.

Four inches of difference is a lot. It's hard to say without a lot more information and detailed pictures, but I would start here:

Does each spring pack have the same number of leafs?
Does one side have a lowering block installed?
Are the shackles the same length?
Do both sides still have the rubber leaf pads installed?
Are the shocks different brands and the correct size for the car?
You said you noticed the difference when you took the subframe off, was there 4" difference when it was installed and sitting on all four wheels or did it only happen after you took the subframe off?

If both sides are identical other than height, then I'd guess you have a few broken leafs on the drivers side (probably more than a few if you are sagging 4"). I would not recommend replacing just one spring. If one is bad, the other is probably on the way out if not already cracked. Especially if they are the original springs. Sounds like a good time for new springs, bushings and pads. :yes:

Good luck!