Next came the interesting part. I pulled up my carpeting and insulation in order to cut away the rusted metal. What had looked like a crack in the metal from the outside was actually the insulation showing through a big hole. This put a bit of a damper on my spirits to say the least. Essentially all of the metal behind the clutch pedal was rusted and a fair amount behind the brake and gas pedals. Wall to wall rust. Fortunately the passenger side didn't show signs of rust. I grabbed a pair of vice grips and started ripping out chunks of rusted metal. Once I had torn most of it back to good metal I had a hole about 8" wide by 10" high, plus a smaller one about 3" wide by 6" tall behind the gas pedal. The "frame rail" was exposed, but fortunately only had surface rust on it. Since this hole wasn't looking as minor as I had hoped I pop-riveted a piece of sheet metal in temporarily, knowing I would want to weld it later.
To fix the hole this way is pretty easy and cheap. It should not cost you more than $20. First cut a piece of sheet metal (1/16" is plenty thick) roughly to the shape of the hole you're going to fix. Make sure you make it about an inch larger than the hole all around. Then get out your hammer and pound it to shape. I was able to re-create the shape of the wheelwell fairly easily. This is an important step or else you may experience wheel-rub. Next cut or tear out the rusted metal until good metal is all that remains. You can do this step before or after you cut your sheet metal to shape. The advantage of doing it afterwards is that you will have more of the wheelwell left to form the new metal to, however, cutting out the bad metal first will give you a much better idea of how big your new metal should be. Once you have the new metal cut and the rusty metal removed, place the new metal in its proper position and drill a couple holes through the new metal and the good metal on the body. Then place a pop-rivet in the pop-rivet gun, push the end of the rivet through the hole, and squeeze away. Once the rivet is tight enough, the tightening bar will break off automatically, leaving you with a secured rivet. You don't have to use too many rivets because they are very strong. If you mess up, or decide to remove the rivet, all you have to do is drill it out with a drill bit.
Then you would want to spray primer on the metal, let it dry, and undercoat it. You can buy rubberized undercoating in a spray can at your local auto parts store. Now you're rust free and watertight!
I tore up all the sheet metal that formed the supports with a pair of vice-grips. I'm sure there's a better way to do it, but that's what I had (a grinder would probably work nicely). This left the floorpan relatively intact. I treated the remaining rusty bits with Eastwood's Corroless Rust Converter (similar to POR-15). The new seat rails were from the black Spider I helped Eli cut up (as seen on the Page). I welded those in place of the old ones. When we cut them out of the donor car we made sure to keep the crossmember which spans from the rocker to the transmission tunnel intact, and to get a fair amount of floorpan surrounding the supports. That way I had plenty of material to work with. It's easy enought to cut the metal down to size, where as it's a bigger chore to fill in gaps or replace rusty floor pan.
Here's another key item: If you're vehicle is suffering from the extensive rust that mine did, tear up all that rubberized coating on the floor. I found it broke up easily by smacking it with a hammer, or using a chisel when it was really stubborn. This will most likely reveal some other rust spots. These I cut out and patched by welding in some scrap sheet metal. The depression behind the seat was all rusted out from water having sat there for years before I acquired the car. This was repaired in similar fashion.