Let's be honest here. I know that there are some welders out there that won't like this, but. Welding is not rocket science. The easiest way to learn is practice, lots of it. Get some pieces of sheetmetal the same thickness as what you need to weld and practice. Weld them together and then break/cut them apart. At first they wil likely break apart at the welds. Later they will probably need to be cut apart. Look at the penetration and the appearance of the welds. Adjust your technique to get the results that you want/need. The main thing with welding body panels is to get penetration without burning thru and doing small sections at a time to prevent warping. Doing small sections also gives you a chance to see how you are doing.
Remember.... Metal, unlike wood, can be put back together, or made bigger, relatively easy. Replacing cab corners or rockers can always be fixed by someone else, if you figure out for sure that you cannot do it. It just may take a little more work.
Just think how you will feel if you do it yourself.