Cant see it being a problem being bolted to box section , i would bolt right through the box section.I think you are supposed to use large load spreading plates under the bolt/nut but 2 points to think about are;
1 If you are mounting the seats on runners this will raise the seat height quite considerably.
2 If you decide to change the seats or runners in the future and cant use the original holes there wont be enough room to slot the holes or drill new ones.
Dunno if that helps much really
