I’m a little late in the game here to offer advise or expertise but here it goes :
My best method of placing in patch or half panels is always use a Air Flange tool. These tools are designed to press the metal edge and allow a clean over lap and then weld in place. You can utube and search videos on good flange tool use and application and it makes the job much cleaner than trying to butt weld panels and trying to manage the heat while placing old panel to a new panel. My advice is to cut out the area leaving about an inch With the old floor and then Start to place your new pans in place . As you get closer to knowing your final fit , cut the panel down and flange press the new panel and set in place . I then set the panel tight with 5/16 sheet metal self tapping screws and drill right through both flanges to make the panel sit real tight. After welding the entire area all the way around with intermittent welds to hold it , I pull the self tappers and fill them in with a plug weld process.
Butt welding floors creates a potential for sagging or a weak area , especially when the car starts to flex when complete.
If anyone needs help Please feel free to reach out to me or PM me and I can add more color commentary if you need help on these types of projects.