Should there be something between gas tank and trunk floor? On the straps?

I don't know how much rust/snow/road salt you have in your area but I would put some paint on or spray on undercoat on the top side of the tank before installation. Of course I would also use the felt material (similar to think roofing tar paper). Lots of stuff ( water/salt/small rocks) end up jammed on top of the tank and can cause corrosion.