audlinz
Since the proportions are different, it will stretch things in one direction or other.
The thing to do, is keep the "constrain proportions" on, and decrease the width to the required size. Now everything will resize proportionally and be ok.
Then, as a next step, go into the document definitions, and adjust the height to the required size (in other words, without using QuickResize).
This will, of course, create some extra white margin, but that's inevitable since the proportions are not the same.
But at least everything will be resized perfectly to fit the new width.
You can then use the free "Snap margins to text frame" script that comes with InDesign (also by Id-Extras!) to adjust the height of the text frames all in one go, if needed.
Does that help?