Add a 1/8" spacer (big washers work great) under the cylinder, where it contacts the lower crossmember. This insures the cylinder is centered vertically. Where to put it is circled in the first image.

Make sure the cylinder is centered, using a speed square pressed against the side of the cylinder, and a tape measure to the to the vertical members as shown.

Make sure the tubing for the pin is square with the lower crossmember using a speed square.