Floor Pan Questions
Don't do any more than you need to do, or over think the process.
It would pay to take a read of this:
Welding basics of basics 101:
http://www.autobodystore.com/new_page_11.shtml
Flange or lap joints:
http://autobodystore.com/forum/showthread.php?20610-flange-or-lap-joints&
For cutting out spot welds:
May I recommend the Dremel 9905 or 9906 carbide cutter for taking out the spot welds, it's the most effective way I've found to do this. Quick n' easy.
7 bucks at Sears.
Mine has done about 65-70 welds, and is going strong. The only downside is it has a tendency to throw metal shards the size of short strand fiberglass. To compensate, I use light oil (WD-40, 3in1, PB Blaster) to keep the shards stuck together, with the side benefit of keeping it cool and lubed. It works very well.
Just be sure to wear: Ear protection, full eyed goggles and/or face mask, jacket and leather gloves, and keep out of the direction it can toss.