Greetings!
Unless it came from the factory as a passenger van, in which case you wouldn’t need any modifications to it, it probably isn’t legal.
There are a lot of safety regulations when it comes to vehicle seating. I wouldn’t risk a life on a possibly unsafe seat installation. I’ve seen such seats ripped right out of the floor in an accident. If the van wasn’t built for seats to begin with, you’re going to need heavy steel plates, both above and below the floor to distribute the load and prevent bolts, even with washers, from pulling through.
Never skimp on safety when lives are at risk.
Cheers!
"Smiles are contagious, pass it on!" ~ Van_Dweller