George Will, by the way, was never a liberal or a leftist. Usually, it's that transition that is a huge part of the distinction. In saying this, Ronald Reagan would fit the description of a neo-con, but I don't know anyone who thinks of him as such. Many isolationist like to think that any conservative who favors a forward defense of the USA as a neo-con, but this is a spin, it seems to me, that's only accepted by those isolationist trying to imply less than stellar conservative credentials onto the supposed "neo-con."
As for your false choice between isolationists and neocons, you are leaving out the realists, and Reagan was a realist, although the neocons did influence him during his first term.
In fact, the Realists were the dominant foreign policy group for Nixon, Ford, Carter, Reagan, and GHW Bush. It was only under Clinton that the liberal interventionists rose to power and the NeoCons rose to power under GW Bush. George Bush campaigned as a Realist but followed the NeoCons.
Some of these shifts coincide with the shift from the Cold War Period to the Post Cold War Period.