A 300mm can make a great portrait lens on 8x10. Just remember it's considered a "normal" focal length on 8x10.
It would be similar to using a 50mm on a 35mm film camera or full frame digital. It actually feels a slightly wider to me due to the 8x10 format being more square. A 14" or 360mm feels more nomal to me.
Jock Sturges used a Fujinon 250mm F/6.7 lens for his early work. Yousuf Karsh used a Kodak 14" Commercial Ektar and I read somewhere that Richard Avedon used a Schneider 360mm lens for his American West project.