Helped a buddy put those type of seats in his 53. We went with larger washers in front under body and in the back. he did go with 6 total bolts. The he looked at me and asked "Think that is enough?" my reply.
"Dude if your body weight pulling on those seatbelts and impact pushing seat pulls that seat out of the standard bolts thru the steel, you got other issues to worry about in your no crumple zone, no air bags truck" He agreed and we moved on to other items.