In anticipation of my '68 being released from paint jail soon I was reading through your assembly post today. I had a similar experience with the rear end being out of kilter. After pulling it all back apart I could find no cause other than the housing had slipped on the polyurethane spring cushions. I wanted to stiffen up the shackle flex so I elected for an aftermarket polyurethane kit. In addition to the shackle bushings, I also installed the spring cushions but left the front spring eyes rubber. Until this build I had always used GM spring cushions and never experienced any problems.
Enclosed photo is what I found upon disassembly so I elected to purchase some of GM's finest as they are still available, part number 3930052 for multi-leaf springs. The aftermarket cushions do not have the molded in metal support for the spring pilot pin like the GM cushions do. I don't know if the aftermarket cushions were the cause or not but after 4 years with the GM cushions the rear end is still located perfectly.
Your problem could be something entirely different. Just something to check. I've never read or heard anything bad about polyurethane spring cushions but mine moved. Nothing else but the cushion was changed in the assembly to fix the problem.
Thanks