Not familiar with the wire brand, but it seems to get good reviews so it's probably just fine -- I was just checking to see that you where not trying to run Harbor freight wire
Most likely it's just you lack of experience -- welding frame stiffeners is tricky
Heavy, 3/16th fresh steel to paper thin, most likely rusted on the inside, tin
Then add in the out of position, hard to see, shit falling all over you places your welding -- it's just tricky
Biggest thing I can add is to be sure you don't hold the tip of the gun to far away from your weld. It's easy to do in that crap position. People tend to pull their gun away farther away just so they can see better -- makes for poor welds
What settings are you using on your welder? What gas flow rate?