Pretty sure you needed to have 'S13-Rallye Suspension' to get a front sway bar. If I'm not mistaken this would have been standard equipment on 383/426/440 R/Ts & 'Cudas and 340 package cars, and optional on others. So something like a 318 or 383-2 would not automatically have one. A car that came with a front bar would have tabs on the lower control arms for it.
I agree not all came with even front bars , as mentioned base cars didn't have them at all , the brackets will bolt into the K frame but the tabs need to be welded to the lower control arms or combined with the shock bolt .