Surly, Which seats are these and what does the connection go the L track look like?The OEM method for mounting the passenger van seat brackets is to have a wide washer/plate that overlaps the flange of the underbody support beam. You don't need to add all that extra bracing if you just do what the factory does. Of course, the factory seat legs are at exactly the right width to drill through the floor and use a plate to overlap the flange right next to the hole you just drilled.
My L-track mounted on floor with stainless fender washers overlapping the flange underneath:
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