You look a little hot on the last weld in the last pic. You may also be moving to fast. Furthermore are you weaving or does the wire come out weaving? If the wire comes out weaving you can try to minimize that by loosining or tightening the tession screw on the drive rollers.
Also how much stick out are you running? I would run about 3/8" - 1/2" stick out on my spool gun. On a lap joint as pictured I would not weave. I would hold the gun around a 45 degree angle (alittle more or less depending on the thickness of that crossmember) and tilt the gun 10 degrees back.
I hope I explained it ok
Also can you get bolts in them crossmembers? If not I would pop a few 1/4" rivets to help secure them plates.