With the pin held in, tack the tubing on the top in place.

Make sure the tubing is pressed up against the cylinder, that is that its as far into the hole as possible.

Youll want to tack it in many places before welding around it, at least 8 on each side.

Tack it as best you can on the bottom by hanging the frame off of the table and overhead welding. Again, insure the tubing is pressed up against the cylinder.

Remove cylinder and pin, as well as the two crossmembers.

Fully weld the tubing on both sides. Jump around a lot to minimize any warping on the tubing.

Block the insides of the tubing so no spatter goes inside. Big washers work well for this.