The 14.00R20 will work on the LMTV.
The Steelsoldiers forum is good place to look around if you haven't found their site before.
Post up your Family of Medium Tactical Vehicles Q&A here.
www.steelsoldiers.com
Just did a quick search after I started replying here shows they have an older discussion about it...
Has anyone successfully mounted 14R20's (14.00R20) on a MTV? I'm pretty sure height won't be an issue but a little concerned about steer tires rubbing. Any input appreciated. Thanks
www.steelsoldiers.com
Hope that helps.