?? Forgive me but I might be confused by the question. Which specific "kit" are you asking about? I can't find anything like that on the brake pages of PY's Firebird catalog.
FWIW, if you just want to restore your factory rear brakes, I'll vote for just using your local auto parts store for a project like this, and skip the repro houses. Everything is readily available, cheap, and your local shop will often give you a lifetime warranty on parts. I very recently rebuilt my entire rear drum setup. Everything was in stock at my local O-Reilly's. $34 got me a lifetime warranty on 2 new wheel cylinders & shoes, and that price included a hardware kit with all new springs & clips.
Plus I gave them another $24 turn the 2 drums. Rarely do drums need replacement unless yours are REALLY badly corroded from being parked underwater for years, or if they have been turned too many times.
Everything else there was just some manual cleanup up of dirt/rust, and lubricating parking brake cables and shoe sliding surfaces, etc. All told it was $58 out of pocket, plus some grease and brake cleaner & fluid, and it only took me a couple hours, working at a methodical snail's pace...and 10 years from now if I have a cylinder go bad or shoes wear out I can exchange it for free...
