The original brackets can found for a lot less. That's what I'd use if I were you. GM numbers 496892 and 496893. Here are some for $119 or best offer:
https://www.ebay.com/itm/75-81-Ponti...gAAOSwk3NdUJ36
I looked up your brackets on Ames, and their description does say they will fit any 60-70's Pontiac v8 to a 75-81 firebird/ta, so they should work. But, sorry to say that they sure look cheaply made in the picture.