Umm, the price I see listed is $279 for the pair.
I paid $70 each for my XV 3 point belts, and they just bolted in using the existing mounts (including the shoulder belt drop, it just bolts into the old shoulder belt anchor), no additional hardware needed. Looks like XV is currently out of stock on their 3 point belts for buckets, but I'm sure they could tell you when they'd be back in stock.
I mean, if XV costs
half as much, something is seriously wrong. And it looks to me like those mounts are not just a bolt in situation, the bracket for the shoulder drop doesn't appear like it would bolt in.
And, on a side note, it doesn't sound to me like your seat belts are stock anyway. My '72 came with retractable lap belts, with that goofy non-retractable shoulder belt that hooked onto a stud on the lap belt. Pretty sure that was factory, both of my '71 Darts are set up the same way.