If I can toss out there, I don't know the exact issues with the Firebird, with the springs being as flat as they are and with new bushings and such I can't imagine the need to a "locator" of any kind unless some VERY serious racing was being done. At the new Goodguys autocross track, you bet your butt there are plenty of leaf spring cars out there with some serious rear axle locators like watts and panhard setups. But with any "normal" hot rod I don't see it honestly. But we all have different images and expectations so ok.
I ran a panhard bar in my truck with leaf springs not for the handling but because I had so little space between the tires and the bed running a 67 Camaro 12 bolt with 8" wide Americans I had about 3/8th inch room on both the inside from the tire hitting the bed side and the outside with the tire hitting the fender. That being said, with the large arch of the stock springs (with lowerblocks too) and the worn out original bushings.
I was running, I wanted the rear end to not move at all. So I wasn't using it for handleing, I was using it to correct a poor design of something else I had done, namely too narrow a rear end for the truck! Now, honestly, I don't remember if I drove it around and then added the panhard to stop it, and it worked. I honestly don't remember how well it worked from without it if I even knew or if I put the panhard from the start of the build, I don't remember. But that was my intentions at the very least, was to limit it's movement as much as I could. I don't have any photos of it in the truck but in the link below I have a few photos showing how I had it mocked up out in my wrecking yard behind my house.
1930u Panhard bar geometry?