Factory bucket seats got reinforcement plates on the floor inside the car. We see examples of the floor sheet metal cracking at the perimeter of that factory scab plate too.
These seats with integral seat belts should be anchored as securely as they were in the donor car. More than a bolt and flat washer through sheet metal at every corner.
Nobody plans to crash but... brake failure, black ice, hydroplane, chit happens. You wouldn't want the weight of the seat to go with you if. Adding underneath the floor could be well worth the effort.
