When I replaced rusted areas of my 66 floorpans, I used a combination of lap, plug, and butt welds where most appropriate. Where there was a fairly straight, flat seam, I lapped it; where there was a severe curve or hard to get at place, the metal was butt welded at those points; and where a panel flange met the inner rockers or frame rails, it was plug welded. Also, I only removed the bad metal which ment trimming the replacement panels to fit the remaining hole, leaving as much of the original metal as possible. For the lap welds, I used one of those hand held pneumatic flanging tools that leave about a 3/4" wide recessed flange where the two pieces of metal will be joined. Actually, with only a 3/4" lap, welding one side practically covers about half the flange anyway. So when you weld the other side, there is very little of the flange that is not solid metal. Its a little tough welding the underneath seam under the car but not too bad once you get the hang of it. I kind of remember having to adjust the wire speed when welding overhead. I ground the excess weld and smoothed the seams which makes the repair hard to detect. I can't emphasize enough to clean all surfaces (top and bottom) of any rust, paint, sealer before trying to weld anything.
66 Fastback
85 GT Hatchback
67 Coupe
82 GT Hatchback