And I think you need to check for better prices on white oak.
Actually air dried may be preferred over kiln dried for a truck bed. Kiln dried can sometimes be a little brittle and if it does take on moisture it can move more than air dried.
You can buy the green white oak boards and in a year or so they'll be dry, faster if you store them inside in the AC and keep a small fan on them.
Really either will work, but I prefer oak and will use it for my flat bed.
Grigg