Bench Seat Replaced with buckets. Brackets/supports needed?

Especially with the repair I would put a VERY LARGE Diameter, thick washer on the stud before the nut on the bottom goes on. In an front end accident the seat tries to pull the nut and washer through the floor pan. Finding a good washer to reinforce the opposite side of the system is cheap insurance. Another thing to do is go farther than the factory that used 4 little spot welds to hold those 4 reinforcing plates in place by welding the plate to the floorboard all around their perimeter. This will help take some stress off the repaired crack. From the bottom you can see the reinforcing brace that takes care of both the bench seat studs and those on the outside of the buckets (that didn't crack).