Is 私はほんとに忙しいでした。 wrong?

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May 27, 2011 12:59 Japanese_language 「です」 「ます」
Sorry for the lack of posts lately, I've been really busy.
私はほんとうに忙しかった。

How would you make it into です/ます style?
If you write
私はほんとうに忙しかったです
it, I think, might be corrected by many native speakers. I would correct it into:
私はほんとうに忙しかったのです
The only difference is “の”. The reason is that the past form of i-adjective + です style is (still) controversial. The reason why I say “still” is on yesterday’s post of mine.
http://lang-8.com/satoshi/journals/944763/

Grammatically, as to this の, one could explain it in various ways; however, I think that the essential reason is the basic strangeness of using an i-adjective + desu. For example, Umegaki Minoru (楳垣実, 1901-1976), a Japanese and English linguist, supported the idea that it should be written as:
私はほんとうに忙しいでした。