.45 ACP uppers do not require a modified lower, but they do require a magazine adapter that fits in the standard magwell. There is a specialized lower called the AR45 that only accepts the .45 magazine without the need for an adapter, but it falls under the heading of nice to have, rather than being a requirement.
Almost all .45 ARs are based on the Olympic Arms bolt, which is blowback and fits in a standard AR15 upper. There are a few custom gas-operated .45 ARs; these use a custom bolt in a standard AR15 upper. To my knowledge, no one has ever built an AR10-based .45, nor is there any need to use the large action.
An AR pistol in .45 would be a custom proposition, but it wouldn't be difficult nor expensive to build.
