Dan the .308 win uppers will not fit on an ar-15 lower. The AR-10 was designed to handle the the .308. Frames and uppers look the same except they are slightly larger. They do share some common parts with the AR-15 The AR-10 platform can handle any of the .308 case family. There is also a Magnum version that will handle the .308 Remington short mag giving you near 308 win mag velocities out of semi-auto ar platform.
The ar-10s are another step up in price range and more difficult to find. A version of the AR-10 design has just recently replaced the Remington 700 (M40) as the Armies Dedicated Marksmen
(sniper is so politically incorrect) rifle.