 Here's the business end of...  Here's the business end of the spot weld cutter, which you should be able to buy at any good auto body supply store. By drilling an 1/8-inch hole in the middle of a spot weld, the locating pin in the center of the cutter will hold the bit in place while it does its job. |  Last but not least on the...  Last but not least on the list of tools (yes, they do perform specific jobs) are safety glasses, ear plugs, and gloves. Get in the habit of using all three of these and you'll wonder how you ever got along without 'em! Seriously! Plus, being able to see out of both eyes for the rest of your life would be nice. |  Here are the '67-79 F-100...  Here are the '67-79 F-100 floor patch panels from NPD. The driver side is on the left, and the passenger side is on the right. Although I won't need every square inch of the patch panels, it is nice to have plenty to work with. |
 Luckily, the rust isn't too...  Luckily, the rust isn't too bad, but it'll still require a certain amount of work to fix. |  To get an idea of the area...  To get an idea of the area of the floor I want to replace, I lightly hit it with the 36-grit sanding disc to make sure there weren't any flakey bits of metal yet to come off. Now I can outline the chunk of floor I'll be cutting out. There isn't much need to cut out an area as big as the patch panels since the rest of the sheetmetal is perfect-that would only make more work than necessary. |  Underneath the floor's toeboard...  Underneath the floor's toeboard area is the stamped sheetmetal piece that makes up the cab mount. This is spot welded to the underside of the floor, but isn't rust and doesn't need to be removed. However, it needs to be separated from the floor and kept intact. Using an 1/8-inch drill bit, I drilled a hole in the center of each spot weld that is in the area I'll be cutting out and followed that up with the spot weld cutter. Be sure to only cut through the top layer of sheetmetal or else you'll wind up with a hole all the way through the floor. Also, to get the most out of your cutting bits, make sure to use a cutting fluid or lubricant as well as even pressure and the right drill speed, because the teeth on the bits are easily broken on the hard welds if you rush it. |
 With a fresh cutoff wheel...  With a fresh cutoff wheel that hasn't been worn down, it's easy to make long, straight cuts in the floor. |  With the perimeter cut, I...  With the perimeter cut, I used a hammer and a sharp chisel to sever any spot welds that may still be hanging on. |  The cancerous metal can now...  The cancerous metal can now be removed for good. You can see part of the cab mount's rear sheetmetal under the area being replaced. By carefully using the cutoff wheel, I was able to slowly cut the floor above the cab mount's sheetmetal without going too far. |
 Next, I enlarged the 1/8-inch...  Next, I enlarged the 1/8-inch holes in the spot welds on the cab mount to 1/4-inch so I can weld the old spot weld holes back up to attach to the new patch panel like it was from the factory. |  Cutting out the right piece...  Cutting out the right piece of the patch panel is important so the old and new floor line up correctly, especially on the ribs in the floor where my right index finger is. |  Once you are satisfied with...  Once you are satisfied with its position, carefully scribe or trace the outline of the old piece of floor onto the new patch panel. |