Good Morning Guys,
I own a welding company for 23 years and have done more floor pans than I care to remember in 30plus years of adulthood.
*Practice on welding on spare same thickness metals before you start if welding is a weak subject for you. You cannot start welding and immediately be a super star. Wear safety glasses always and a welding shield and good leather gloves. On TV you see people all the time welding with out safety gear and is stupid. Watch U-tube for welding in body panels for visual guidance help as well.
*First off- remove all the interior up 24" from the welding area- anything you leave in the car including carpet can continue to smolder for days before it burst into flames. All interior removed is recommended for safety reasons.
*Fuel lines and fuel tank removed or at least safely capped off is recommended. Fire extinguishers (at least 2- one near the car near where you are working and one near the doorway if someone needs to come in if there was afire. I know, I know - its a bit over kill, but safety is not laughing matter. Cover all glass with welding blankets or thick cardboard and good duct tape. Glass is ruined so easy by hot sparks.
*Remember to remove anything that cannot withstand up to 1200 degree burns. some of the molten metal and rust / slag get up to 900 degrees.
*If you cannot cut the spot welds with a spot weld cutter like a Blair rotary cutter bit. just flat grind over the weld thru the metal you want to remove with a hand grinder.
*Use a "whizzer" or otherwize known as a cut off wheel to cut the majority of metal out of the way leaving only the frame rails.
*The new metal needs to be cut and shaped a bit because they are never perfect.
Line up the panels and pre drill like 3/8" holes where the old spot welds are and using a MIG welder, fill the circles thru the base metal (original frame metal) and make sure the two pieces of metal (original frame and new floor) are as tight together as possible.
*seal your joints like the factory did- dont use silicone... it does not last. 3m makes a good paintable seam sealer like $7.00 for a caulk tube size and can be bought at like napa or auto body supply company..
Good luck and hope I helps some.
I commend you on trying this process and going out of your comfort zone. Good for you!
Joe