A smaller pair of tabs was built, also from 1/8-inch plate, to mount the upper arms to the rear axle, just to the outside of the third member housing. These arms are fastened with a bolt, and will be plenty strong to handle the longitudinal force.
Note the section of 2-inch square tubing stuck between the axle and the C-notch in the frame (arrow). These were used on each side to establish the proper ride height and pinion angle while Ryan accurately located the upper and lower arms. The mounting tabs at the front and rear of these upper arms were sized to keep the front and rear pivot points horizontal (and the arms parallel to the ground) at ride height.