I don't care if you attack the sources I cited. I only cited them because they were the first three links that popped up in the Google machine when I typed "Marshall Plan Keynes." Yell at Google, not me. It's not cherry-picking if you aren't picking.
http://lmgtfy.com/?q=marshall+plan+keynes
I skimmed the DeLong article. My reading would indicate that he assumes a Keynesian construct for the whole thing. (DeLong is a Keynesian in good standing, so this is unsurprising.)
What DeLong says is that the Marshall Plan did more for the European economy than one would expect from the relative size of the stimulus, and proceeds to examine why that appears to have been the case. An implicit assumption is that stimulus (i.e. Keynesian economics) works.
"Marshall Plan aid of two and one-half percent of GDP goes a substantial way toward closing [the] excess demand gap." (at 46)
So, basically, pump more cash into an economy which is sputtering because of poor demand, and demand increases as persons with more cash spend the money.
The basic concept is Keynesian. The economic impact of the Marshall Plan was disproportionate to the stimulus, but in a good way, which led DeLong to ask what other factors were present.
So, I need you to explain to me how the Marshall Plan was not really demand-side stimulus (even though the DeLong article you cite seems to think it was) or how demand-side stimulus is not Keynesian.
Keynes wouldn't have cared how the money was spent, whether you dropped it from a helicopter or (as in the actual Marshall Plan) mandated that governments match Marshall Plan aid dollar-for-dollar (or lira, franc, mark, whatever) to be spent on US-approved projects. Because, you know, in the long run we're all dead.
The fact that Colonel Klink helped re-vamp the West German economy and gave it a more free-market orientation is great, and I think it was very significant. But neither 1955 West Germany or 2015 unified Germany is terribly "free-market" by the standards of Anglo-Saxon observers. Their mix of state and private economic actors is much more "statist" than is ours or England's.
Government spending to stimulate demand = Keynes
The Marshall Plan spent US Government money to stimulate demand in Europe (and also help them reform their economies and keep them on our side of the Iron Curtain).
Win-win-win.
It was awesome.