My Dart Swinger Project!

I would make that V notch 45° on both halves giving it a 90° V when put together. This will make sure you get full tube wall penetration, and full strength. That will help a lot with weld penetration. Also Your welds look like they are either too cold, or your running the wire feed too fast. Slow the wire down, and bring the temp up. If you follow my advice you can grind off the excess weld and make the tube look like it's never been cut.

Clean off all that scale on the other end of the tube at the pumpkin end to make sure your jig is gonna keep it as square as it can. That lumpy, bumpy rust will throw off the squareness.

V notch that practice piece 45° on either side for a full 90° V slow your wire down and turn the heat up. Your welds look cold. They should flow into the metal not ride on top of it.