We usually suggest to new members with (new to them) 1st Gen Firebirds to order a copy of all the original factory documents that will let you know exactly how your car left the factory.
For instance, dual rear axle traction bars were never installed on cars that left the factory with automatic transmissions (1967's only). I did retrofit my built in California 400 '67 with both bars. One of the things that is the most difficult is to overcome the fact that the factory welded on a reinforcement plate on the top of the rear floor pans to support the front bracket for these bars. If you pull the rear seat bottom out on a factory auto trans car, you will find a reinforcement plate only on the passenger side.
Sorry but I can't comment on the Hotchkis pieces as my rebuilt my rear suspension close to factory stock (except for the 2nd traction bar). I can imagine that rear wheel "hop" would be pretty bad with the single leaf springs and no traction bars along with the big Chevy motor. Now you know why Pontiac retrofitted the axles with those bars.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI