I suggest you consult an alignment shop that deals with high-performance vehicles, which is what you have. A local Chevy shop may not be good enough.
I remember having an alignment tech tell me, "The factory recommends this and that, but we've found it works better with such and such." I said, "OK, give me such and such," and the car was TRANSFORMED from a wallowing pig into a car I could actually steer. A few minor adjustments made all the difference, and I think that's your problem.
You need to find an alignment shop that understands you car.
Rear suspension? It's a solid axle, right? I wouldn't worry too much, but have the thrust angle checked, just to be sure.