Thanks 19silverado77. Sure I can give you tips, but fair warning, this project has been my first experience welding sheet metal as well.
First tip is the google search box at the top of the screen is your friend. Type in "welding patch panels" and select the "67-72chevytrucks.com" dot and you'll get links to a ton of threads specifically on this. Here's an example of a thread I posted asking for this same kind of advise myself.
http://67-72chevytrucks.com/vboard/s...d.php?t=576708
I can also give you a few tips that may not be spelled out clearly elsewhere. When you buy your patch panel, it will likely be much larger than the rusted area you need to patch. You don't need to use the whole patch and probably don't want to for these reasons:
1) Smaller patch means less weld which means less opportunity for warping (but you'll probably get some anyway).
2) The leftover patch material is good for those places where you have to make your own patch. I have more of these spots on my cab than the spots I was able to get a patch panel for.
3) Most important, by cutting the patch down to just what you need, if you screw up you can cut the patch and a little more original metal back out and a new patch will still fit for a re-do.
I probably spend twice as long getting a patch to fit as actually welding it in. If you leave gaps that are too wide, they are tough to weld and generally in my case make the job of grinding, fixing burn holes, etc. bigger than it should have been. If your best effort at fitting the patch still leave some slightly too big gaps, use a copper backer behind what you are welding to help you bridge the gap. Weld won't stick to it, and it absorbs heat helping you not to have burn outs. I use a piece of copper tubing I smashed flat with a hammer.
When welding in the patch, everyone will tell you to take it slow. Make a few tacks to hold the patch in place and then fill in the gaps a little at a time, letting the metal cool before you go back to the same area of your patch. One piece of advise I got was rather than putting tacks in the middle of other tacks, start each tack on the edge of another to help prevent burnout. I'll only put 2 or 3 tacks in a row slightly overlapping the last one and then move to another area. Some may say even that is too much in one spot.
Grinding is where I probably have made the most mistakes. In my own experience, when I do a butt-weld for a patch panel, the metal shrinks in a way that leaves the weld in a slight valley. Not a problem if you have access to the back of the weld because after grinding the weld close to flush you can use a hammer and dolly to flatten the area before finish grinding. The mistake I made was attempting to grind the weld flush when I couldn't hammer it back out and didn't really realize what was going on. I was taking off too much metal on either side of the weld. Then, when filling pin holes I'd end up burning a hole in too-thin metal. I've actually had to cut some of these areas out and patch my patch
The technique I've adopted lately is to take the weld down near to flush using the edge of a cutoff wheel, just moving back and forth on the weld. I have better control than using a grinding disc because for me it is tough to see just what is being ground (likely metal beside the weld) using the grinder. When you get it close to flush, I've had pretty good success using either a flap disk or a 36 or 50 grit roloc disc on a die grinder to finish taking it down.
For all that, I still don't get patches that look as good as many examples you'll see on this forum. I will be using some "kitty hair" filler over my butt welds. It is supposed to resist cracking better than regular plastic filler. It is also waterproof so it will hide the ugliness and fill whatever gaps I left in my weld.
Hope this helps you some. Main thing is read as many of these helpful threads as you can first, and then practice on something else before you jump right into your fender.