As far a size, to each frame that is already built, you must custom fit that cross member to your frame... and if you use only one tube for a radius rod/control arm, I would use at least 1 and 1/4" OD X .188 DOM wall min. My friend uses 1 3/8" OD x .156 wall Mild steel, DOM... Or you can make them look like regular double tube radius rods like used on a straight rear end, if you have the room, but still use the single pivot at the rear end mounting point.