When i did my old tinny. I pop the rivets out of the bench seat.
Installed the slide in with grade 8 bolts,Over size washer and loc-tite.
Then pop riveted the bench back down.
this took a couple of hours, but I know that seat has not move.
Nephew has the boat now.
When using rivets, make sure you use the right size.
To large of a rivet will spider crack around the holes, loosening the rivets.
Don't know of any thing that will hold down the seat. By just screwing it down.
For How thin the aluminum is in the bench it will come loss.