My daughter saw these benches at a flea market and she wants me to build
one for her:

https://i.imgur.com/5n5kB3K.jpg

The bench she wants will be 5' long. The problem I foresee is that she
rents the first floor of an older home (1940) and the wide plank floors
aren't level/flat.

Am I nuts to be wondering if there is a way to build the bench so the
overall look is maintained and the bench is sturdy enough to hold 2-3
adults, but has just enough flex to not rock if the floor has a dip or
sag?

Maybe with 2-3 adults it will flex on it's own even if fully glued and
bolted, but what if only one person is sitting on it?

Am I, as usual, overthinking a simple project?