Rear sway bar suggestion

If you're planning on keeping the stock front bar, I would try and find a stock rear sway bar as well. As already mentioned, having the rear bar larger than the front isn't a good idea unless for some reason you've set your car up in a way where that works.

While the torsion bars in these cars were greatly undersized from the factory, the rear spring rates were actually pretty close to where they need to be for performance handling. Much closer than the front. A-bodies in particular tend to oversteer with a rear bar, depending on what springs you have out back a rear bar might not even be necessary.

What size torsion bars do you have in the front? Do you know what leaf springs you have in the back? That will go a long way to picking the sway bars you need.