If your seats use a threaded stud through the floor, I would highly suggest trying to use that to mount the base plates of the chair bases. You can also make adapters out of sheet metal for either the seat to the base and/or seat base to the floor. Reason I suggest that is the unibody frame rails get int the way underneath so drilling new holes may not be easy unless you’re using lag screws, which I personally wouldn’t trust the strength of heaven forbid there’s an accident.
If the van has the original hood, it should say what transmission it has, my guess is a standard 727
