With a table layout you cannot fill 2 columns dynamically. You need to use floating DIV's. This should work (not tested):
<div>
<!-- BEGIN news_block -->
<div>
<div>{GROUP_IMAGE}</div>
<div><strong>{TITLE} {POSTED_WHEN}</strong></div>
<!-- BEGIN readmore_link_block -->
<div style="width:50%;float:left;">
<div>{CONTENT_SHORT}</div>
<div><a href="{LINK}">{TXT_READMORE}</a></div>
</div>
<!-- END readmore_link_block -->
<!-- END news_block -->
</div>
</div>
Of course it's better to style the DIV's in the stylesheet, instead of inline like here. You will also want to layout them, because now the columns are right next to each other. So you need to apply margins, and maybe background colors, borders, etc. to make it look nice.