If you keep the person centered, as in the nose in the center and more "sky" over their head, you can minimize distortion and use a relatively wide 300mm. It's the edges that give you away, you see this with a lot of amateur shots where they used their wide angle zoom on their dslr to fit the entire person into the frame. You'll often see a beautiful svelte young girl with size 22 feet at the edge of the frame ;-p

A 14" Commercial Ektar or 360 Symmar is a great all-around people lens for 8x10, especially outdoors in natural light where you don't want a huge bellows extension to become a sail in the wind and you can use every bit of fractional speed possible over the longer lenses.