My only concern is load bearing. If there is already a floor and the planks are just covering a floor already down, such as a plywood floor, then I think you’re ok. Cedar is not known for its ability to carry a load.
It is also notorious for splitting when anything drops, strikes or otherwise stresses it. My fence for instance was bumped into by my landscaper and it caused the gate to split several planks.
Polyurethane would work fine IMHO but a better more durable finish might be varnish or something with a little more strength.