I've attached some pictures with my post. The rear of my 72 cutlass convertible is sagging on the passenger side. I've replaced the coil springs and the shocks over the weekend and it helped but still has a noticeable sag.
The first picture is of the passenger side spring perch,The second is the drivers side and the third is the way the car sits.
Why are the coil spring perches different? Could this be causing my sagging?

 

I don't believe the perches are your problem, unless they are way off centerline where they are welded to the tube.

Most likely your body mounts are squashed down into the frame holes, or your upper spring pockets have stretched over time.

Mine does the same thing. Lopsided as all hell.
I looked everywhere for frame cracks, and found none, so I just stuck an airbag in one side.

 

Are the new springs turned the right direction? Think the CSM says pigtail points out but don't have the book handy.

 

Your problem can also be a weak spring in the front! Measure all 4 corners, a weak spring on the front right will cause the rear left to go up, same with front left and rear right!

 

I have the same problem on my 67 GTO, I tried everything changed coil springs a couple times,checked front coils,checked body mounts, had friends look at it thinking I'm missing something?? I went with air shocks with seperate lines to get it to sit even. Worked for awhile, but air shocks are a pain in the a-- They never hold air very long, always adding air!! so I went and got the coil spring speaders that insert into spings, they also can compress the coil sping. You can get them at local auto part stores. I compressed high side a little, Then I lifted low side to even car out. Work out perfect cars been sitting even for the last8 years !! LOL Hope this helps you, Good luck!!

 

Wow! Just this weekend I looked and saw the passenger side rear of my 71 cutlass convertble sagging too. Didn't do it last month! I'm keeping an eye on this thread. steve e

 

Are the tires the same size side to side? They are different tread patterns and even if the same size, different brands have slightly different measurments.

 

The frame rails have a tendency to roll in the front . Find a good Body shop with a frame machine to get it checked

 

Keep your mother-in-law outa the car!! [haha]
Try switching the rear tires - I also noticed that!