The aftermarket seats I've had have come with rails underneath which you would bolt to a plate like Frscke1 said. If you already have pedestal type seats, they may already have the plate.
When you go to remove your current seats, you may find that if there is a slider system to allow fowards/backwards adjustment of your seats, the sliders could be swapped directly into your new seats, which probably don't include sliders right out of the box. If your seats don't currently include sliders, this would be a golden opportunity to install some. They're ridiculously inexpensive, like $15 or so per seat.
Be sure to use decent quality bolts to hold seats and mounting plates together.
-It's been such a LONG TIME... BlueShift>>
1981 Dodge Ram B250 Custom Sportsman Maxi Van
It's what you learn after you know it all, that counts...
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