Floor patches should be butt welded if you're doing a quality restoration. That retuns the car closer to factory original condition than a flange welded patch. It takes longer because the patch needs to be accurately fit. For a first generation Firebird, it's worth taking the time.
If a flange welded repair is made, it should be seam sealed so it won't rust. Some seam sealers are not intended for use over bare steel, so epoxy primer is needed first with those. Check the package insert for the sealer product you use.
