I have a complete set of them still in boxes, I haven't tried them though, they were kinda pricey, close to $400 If I remember right, but they are trick, adjustable,tuck up under the car nice, and don't tearup your leaf springs like slappers do. I have a broken leaf on my car now from slapper bars. I am going to put them on when I go through the whole car. I have been accumulating parts for almost 5 years now. I learned the hard way on restoration/rebuilds, buy everything you need before you tear it apart or your project will stall and become a pile of scattered dreams.
Well, as my luck would have it, I'm a day late and a dollar short. I ordered my bars and springs from them yesterday!!!! Noone responded to this thread, so I just went ahead and ordered them. Thanks anyway, guys.
FYI, the springs are a "split" monoleaf design, and they cost $258.00. According to Travis at Calvert, I bought the last ones in the warehouse. It will be 4 weeks before they get more in. As for the bars, they have tons of them in stock, and I bought mine with the '67 Camaro shock plates. It mounts both shocks inboard, giving more tire clearance. Good luck.
It might have been Travis I also talked to a few years back when I ordered my set. I got the staggered shock plates for my 71 Z28 and they also came with sold aluminum leaf bushings for the front. I didn't get any springs from them though, maybe they hadn't made them yet.