Was it welded around the entire edge or are there gaps between the welds of larger than 3" or 4". If there are large gaps between the welded areas it would need to either be completely welded or you could leave 1" of over lap and plug weld/ roset weld for strength. 5/16" is the standard size plug weld for panel replacement. Either way it should be seam sealed before under coating or painting. Seam sealer comes in tubes squeezable/cualk gun cartrage or cans of brushable which works best with an acid brush. If the floor pan section is the only area that would need seam sealing I would recomend a sqeeze tube. This should be available at any auto parts place that carries auto paint supplies.
Electric angle grinders with cut off wheels cut sheet metal very quickly so I would score the metal in small sections that would be able to be bent on the scored edge to break off the unwanted sheet metal (like peeling an orange).
Electric angle grinders with cut off wheels cut sheet metal very quickly so I would score the metal in small sections that would be able to be bent on the scored edge to break off the unwanted sheet metal (like peeling an orange).