I know the exact feeling, I went through the same thing when I did my 70 Challenger back 2000 - 2002. You are going about this in exactly the right way, slow, careful and precise. One thing I can tell you reference MIG welding; do not give into the temptation to run a bead ever (as in a continuous weld) not matter how much there is to weld up or how thick you think the material is, rather use a series of short spot welds (called stitch welding) and move around the panel that needs to be welded. I am sure someone will jump on here and describe how they ran short beads and didn't have any problems and that is possible, but you run the risk of putting too much heat into the panel and have the panel warp which will cause you even more pain. You don't have to ask me how I know, I will tell you that I did this many times before I pulled my head out and stuck to stitch welding.
Do not be afraid to stop and knock down some of the spot welds with a flap wheel once in a while but be careful not to put to much heat into the panel with the flap wheel either. Having a air hose available is a huge benefit in controlling the heat; make a few welds, blow on it with the air hose a little and then back to welding, blow some air onto it, knock down some welds, repeat over and over. It is slow and time consuming but the results will be worth it.
Also, sometimes you can weld a panel or part of a panel, grind it down and get it to look like it was never welded; this is the exception rather than the rule. We all want it to look like this all the time, but it just doesn't work like that. You have to learn to accept the fact that some (most) of your welded panels will not come out looking like there was never anything welded there. This was a problem for me, I had it in my head that I could make this happen and tried very hard to get it, the result was that in some cases I ground through the panel I had just welded... Also I got some very thin panels which I then had to go back and fix, again... The key is to get the joints welded and knocked down to a point where a small skim coat of body filler will cover it (thin skim coat as in less than 1/8th to 1/16th inch).
One last thing reference welding, the distance from the tip of your welder to the panel you are welding is important, practice getting the right distance that delivers a good weld with penetration and has good gas shielding. You need to learn the trigger and the sound of a good weld; it should sound like bacon sizzling on a pan, if it is spitting and sputtering it is not penetrating. Remember to cut the end of you wire off before you start welding, even just a little bit because it gives you a clean wire to start the weld verses the typical ball of melted wire which is dirty and sometimes doesn't like to start. Whatever you are welding needs to be relatively clean, MIG will weld through a lot more crap than TIG will, but to get a good clean weld you need to the 2 panels/parts relatively clean. The backs should be relatively clean as well near the weld, like on those quarters there is undercoating or sound deadening on the inside and that will both burn and get sucked into your weld.
Sorry for the long response, but I learned all of this the hard way. It is not hard, just takes patience and a little thought and you will be good.
