Trunk floor pan

If you are going to learn to replace a welded body panel, the A body trunk floor is about the easiest panel to replace.
For removing the old one. Here are some tips. I won't get into all the cleaning, painting, weld through primer stuff. Drill the spot welds at the lower edge of the tail panel with a drill bit. Helps to use a 1/8" first, then go up to a 1/4-5/16 to drill them apart.
At the rear crossmember and front edge of the floor, use a spot weld cutter and carefully drill through the top layer only, or use a thicker cut off zip disc and carefully "erase" the weld by grinding away only the top layer of the trunk floor, at each spot weld. I like this method to seperate the welds on the frame rails. You can use the zip disc to or an air chisel to cut just inside of the trunk extensions and then remove the leftover flanges once the old trunk floor is out. Once it's all out, dress the old welds etc to get smooth flanges. Drop the new pan in place and check its position. I go underneath, and mark with a paint pen where every original spot weld was located, and trace out the rails etc on the new panel, so I can place the new plug welds in the same spots as the originals.
With the new floor upside down and marked, I drill an 1/8" hole at every plug weld spot. And then you can put the new floor in place, use about 100 or so drill screws to hold it to the rails etc. Then once it is fully screwed in place, begin removing alternating screws, drill a 5/16 hole through the new floor, and let it countersink into the frame rail below a bit too. This gives you a clean, strong weld.
And make sure your welder is set hot enough to get good penetration. There's a lot more to it than this, but hopefully that gives you an idea