Looking for seat rail bolts for my 67 coronet. The bolt that goes thru the floor pan. Thanks

 

my factory paper service manual show a stud pressed in to support that goes throu the floor pan with a nut with a captured washer, on the outside bottom. nothing on size. if it was mine i use min 1/ 4 bolt grd 5 and large washers and lock washer. but not much detail from the factory