Ron's right, you only have one U-bolt and two T-bolts on that side. Since the rear axle assembly and traction bars are not original to the car, would it make any difference getting the exact same springs the car came with? Unless you plan on restoring the rear to original in the future? Looks to me you have five leaf springs, the bottom spring is always shorter on the one end than the other but yours looks like the second from the bottom is also shorter at the back, usually goes out past the spring clip. Maybe modified when the traction bars where added?

Some guys have a lot of trouble getting the stance they want when installing new springs, trial and error is a real pain when it takes so much work to remove and install. I did my front springs multiple times. I lucked out with the rears but I have the back a bit higher than others like.

Whichever spring you get, if you aren't worried about originality, I'd replace the T-bolts with U-bolts. I don't know why GM thought it was a good idea to use T-bolts but seems to me U-bolts would be a lot stronger. Maybe something to do with the brake lines, who knows, but bolting the springs to the perches instead of wrapping around the axle housing tube seems a bit lame to me. The weld attaching the perch to the axle tube on one side of mine was cracked, so I re-welded it and I replaced mine with 1/2" U-bolts.