i have a rear end question!

A lot of a-bodies are not straight in the rear . Check your housing for squareness. If the rear pulls to the left shim the right side back . Opposite for a pull to right.

The front spring perches were usually off on open rear cars. The 340 cars were close. The bodies in white were perfect. My Duster was off a 1/2 inch it was a /6 back half. My 340 Duster on the rotisserie is off 3/16. I have shown this to several members who came here and they found theirs were off as well.

Did you ever watch cars in the burnout box and they can't keep them straight. This is why. I would say this is more than likely your problem. See it on many cars that come here for this same problem your having. They were built this way.

I use a aluminum shim in front of the the perch with with extended bolts if needed.

How even do your tires sit in your wheel house? Some times it is noticeable there. But a measurement from the rear K-member bolts X'd from side to side will tell the story.