Mark the location for the hole for the pin on the bottom horizontal member using spray paint.

Put the top member on, and put in at least 3 bolts. Spray mark this location as well, from the other side.

Remove the horizontal members from the frame.

This is possible to do without removing the cylinder, just prop it up on something to remove the lower member.

Place one of the DOM tubing pieces in the center of each of the sprayed marks and spray paint around this to mark where you will torch.