Where I have straight sections of overlapping joints, I've been using galvanised bars and steel flats to sandwich the original floor or sill metal against my replacement galvanized repair sections. It's probably overkill, but hey, as long as I have the time and I'm already in there.... And bolts are often easier to deal with than wrestling with a rivet gun under the car or in tight spaces, especially where there's a slight gap and I need to draw the original sheet-metal and repair sections together.
On the other hand, where there's minimal structural impact, involving a few individual rust holes up to half dollar size in a floor or panel, I'll sometimes cut small thin galvanised patches, then glue them in place with spray-undercoat.

Happy Motoring, Mark

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DrDKW