A body with B body rear...Sway bar?
I'm working to improve my rear suspension even more. I have a 68-70 B body rear with a 1/2 inch spring relocation under my 70 Duster. I have the front suspension pretty much dialed in with the exception of shocks, this goes for the rear as well. I think I've settled on the Hotchkis/Fox shocks. I'm looking at doing some autocross and am wondering if I need a rear swaybar and if so, which one. I have stock leaf springs which seem to be OK, but maybe there isroom for improvement there. I've been seeing posts about Hotchkis leaf springs and thinking maybe with them and the shocks, a sway bar may not be necessary.
The best your stock leaf springs could be is 110 lb/in, those were the XHD's. But more than likely they're either 90 lb/in or 100 lb/in. In either case that's going to be a bit on the soft side, most of the autoX guys are running ~130 lb/in springs (Eaton from Bergman, Hotchkis, etc). I have a set of AFCO's that are 120 lb/in, which I run with a 7/8" E-body Hellwig sway bar and hotchkis shocks. Pretty much everyone doing autoX on a regular basis runs a rear sway bar with those springs (as well as a front bar).
As for which sway bar, the Hellwig and Hotchkis A-body rear bars are both good options, although it doesn't look like the Hotchkis rear bar is currently available. The Hellwig is a little easier to install, you can do it without welding anything where the Hotchkis bar has hangers that weld onto the frame rails.
The A-body Hellwig bar is 3/4" and should be just fine. I went with the E-body bar because of the 1/2 offset I also have on my 68-70 B body rear, but the way it mounts it still puts the end links at a bit of an angle. I have a set of heim jointed end links to solve this issue, but really just staying with the A-body bar is probably a better bet for most folks.
The other thing is that you do want to keep the car balanced, so, you will want a front sway bar if you don't already have one, and your front torsion bar diameter is also an important consideration. As well as your tires. If you're running the usual all season radials and stock-ish sized torsion bars up front, you won't want to go crazy with the rear leaf spring rate either. If you're running wide tires with a sticky compound and large diameter front bars, you'll need the higher rear spring rate too.