Here is my TextBlocks
It looks like xml:space="preserve" should do the trick (see Preserving Whitespace in XAML) but that doesn't seem to be working in a Windows Store app (it does in WPF).
If you use the non-breaking space character it does work
I suppose you could try building a converter on the Text property to check for trailing spaces and replace with non-breaking spaces - presuming the truncation that's happening doesn't occur too early.