I don't know if anyone reproduces those. When the bodyshop replaced my whole floor, they cut out around those additional braces in the 'vert and ground off the sheet metal. When reinstalling, they put the one-piece floor back in and plug welded it like the factory.
Todd
69 Firebird Convertible, Crystal Turquoise Metallic, Parchment Interior, White Top. Fold-down back seat.