Rock On ! You will be more than a little proud when you load-up the fam and drive that monster around town. You will never loose your vehicle in a Mall parking lot again !
Welding: Welding has a few variables that need to be tuned- the amps on the knob, how fast the wire is fed, how fast you move the tip, and how far the tip is away. The best way is to use some of those cut-off scraps and keep fooling with them until it looks perfect. Your using the exact same thickness scraps to dial-in your settings. When your learning, you will get sputtering and it'll clog-up your tip. Make sure you keep the tip clean- they sell a cool needle nose plier tool just for the purpose, but any needle nose plier will work in a pinch.
Your welds look a little hot or your feed is a bit low. Take your time fitting the panels and eliminate any gaps best you can. When you prep the 2 edges that are to be butted, grind a little bit of an angle so it forms a v-canyon ( ____/\____ ) and you can fill the void with your bead. Your tack/stitching approach is good as it reduces warping from too much heat. Fool with how far you hold the tip from the surface- you can angle the gun and drag the fat outer shield part of the tip along the metal to keep the tip to surface distance constant.
I never had any formal welding training, but learned just like you. Practicing on scraps saves time cleaning-up w/ grinder wheel. If you have big gaps, it's best to tack a strip of steel behind it to overlap, then fill otherwise it tends to keep burning through.