Contact Dick at Firm Feel. I am running their above axle frame hung 3/4” rear bar and love it. The rear bar should be smaller than the front. With 1-1/8” front bar a 3-4” rear bar should be perfect.
That is good advice.
I had a set of the Addco bars on my '70 Charger. 1 1/8" solid front, 7/8" solid rear, As stated, their hardware and mounts are CHEEZY.
What I guess you are looking to achieve is less body roll/lean and a neutral cornering condition. The size of the bars is totally dependent on the weight of the car, torsion bar size, leaf spring rate and don't forget...An aluminum headed LA series engine weighs about 200 lbs less than an all iron 440. The battery up front or in the trunk matters as well.
My current setup is 1.15 torsion bars, 1 1/4" solid front bar. MP XHD leaf springs, 3/4" rear bar, Bilstein shocks, frame connectors, torque boxes.
You may be looking for a simple answer but to get this right, you need to consider the overall package.
When I had the Addco bars, I had 1" torsion bars and the 1 1/8" sway bar up front and the 7/8" bar out back. The car was twitchy and wanted to oversteer (Spin) when I tried to corner hard. That is a decent setup for an autocross car but dangerous for the street or road course. A car that oversteers only gets worse the faster that you go. Imagine a fast maneuver on the freeway !
My original combination was not fun or safe. When a car oversteers, the fix is to either add more spring rate to the front or reduce the spring rate to the rear. You can alter spring rate with sway bar size or torsion bar/leaf springs.
I first tried removing the rear bar. The car had more body roll but wasn't oversteering anymore. I tried adding a 3/4" bar I pulled from an '83 Imperial. The oversteer returned. This told me that to maintain an acceptable amount of body roll/lean, I would have to really step up the front spring rate. I went from 1" Torsion bars to 1.15" and went to an 1 1/4" front sway bar. Now the car lays flat and has slight understeer condition.
Finally, pay close attention to wheel offsets and widths and tire selection. I run an 18x9 front wheel, 4 3/4" BS and a 275-40-18 Nitto NT 01. The rear is an 18x10 with 4 3/4" BS and a 295-45-18 Nitto. A wider rear tire will reduce oversteer a slight amount.
