Jim, yes the .243, .260, 6.5CM, 7mm-08, and .308 all can be switched out on a single lower for most brands. Some differ such as RRA's LAR they use their own magazines so they have their own lower. The .243, .260, 6.5CM, 7mm-08, and .308 all use the same BCG and bolt face. The only manufacturer to date that i know of with a AR-10 lower that is compatible with a 5.56 upper is Colt, they released it at SHOT this year and it requires Shims to do so. But it would be a useful lower for sure.
If i was looking along the lines of an AR-10 it would be in the direction of KAC, LWRC, POF, and LMT. Most of those are expensive but they're all reputable and very accurate. The one i plan on getting in the future will be KAC, just because i love the M110 package. But the LWRC REPR is also very nice and has a side charging handle.