Two points. First, the tabs on 73 and later bars were inboard so the calipers could be front mounted. But as you noticed, the 73 bar will not fit an early A-Body. Second, most if not all aftermarket early A-Body sway bars come with a 90 degree bracket which mounts to the lower shock bolt. This is where the end links go on the aftermarket bar. You will probably need to rear mount your calipers with this setup. I would mock everything up and at least tack that bracket to the LCA before you paint. Also consider welding the bar mount brackets to the K-Member as well once everything is lined up.