Well Bandit,your bumper shouldnt fall off going down the road for now, and if you get smacked in the rear it will only be pushing it together, I would think its fine.,,

The tube welds should be ok, but what you could do is slow down the wire speed and move in a wider zig zag to get the smooth fillet shape you want,,,you should have the same shape weld regardless of stick or mig welding.

And when you are welding different thickness metals spend more time around the thicker metal, it needs the heat to penatrate , the thinner stuff will melt faster,,,either do this by slowing down as you pass the thick metal,,,or just move your whole zig zag closer/over the thick metal...also I made a reply for directions to Johnson vally in the other post.