easy:
In the forumdisplay_threadslist template, find:
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<td bgcolor="{tableheadbgcolor}" align="center" nowrap colspan="3"><smallfont>
<input type="hidden" name="s" value="$session[sessionhash]">
<input type="hidden" name="forumid" value="$forumid">
<input type="hidden" name="pagenumber" value="$pagenumber">
<a href="$sorturl&sortorder=asc&sortfield=title"><font color="{tableheadtextcolor}"><b>Thread</b></font></a> $sortarrow[title]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" nowrap><smallfont><a href="$sorturl&sortorder=asc&sortfield=postusername"><font color="{tableheadtextcolor}"><b>Thread Starter</b></font></a> $sortarrow[postusername]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=replycount"><font color="{tableheadtextcolor}"><b>Replies</b></font></a> $sortarrow[replycount]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=views"><font color="{tableheadtextcolor}"><b>Views</b></font></a> $sortarrow[views]</smallfont></td>
$threadrating
<td bgcolor="{tableheadbgcolor}" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=lastpost"><font color="{tableheadtextcolor}"><b>Last Post</b></font></a> $sortarrow[lastpost]</smallfont></td>
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And replace it with:
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<td bgcolor="{tableheadbgcolor}" align="center" nowrap colspan="3" width="45%"><smallfont>
<input type="hidden" name="s" value="$session[sessionhash]">
<input type="hidden" name="forumid" value="$forumid">
<input type="hidden" name="pagenumber" value="$pagenumber">
<a href="$sorturl&sortorder=asc&sortfield=title"><font color="{tableheadtextcolor}"><b>Thread</b></font></a> $sortarrow[title]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" width="15%" nowrap><smallfont><a href="$sorturl&sortorder=asc&sortfield=postusername"><font color="{tableheadtextcolor}"><b>Thread Starter</b></font></a> $sortarrow[postusername]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" width="5%" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=replycount"><font color="{tableheadtextcolor}"><b>Replies</b></font></a> $sortarrow[replycount]</smallfont></td>
<td bgcolor="{tableheadbgcolor}" width="5%" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=views"><font color="{tableheadtextcolor}"><b>Views</b></font></a> $sortarrow[views]</smallfont></td>
$threadrating
<td bgcolor="{tableheadbgcolor}" width="25%" nowrap><smallfont><a href="$sorturl&sortorder=desc&sortfield=lastpost"><font color="{tableheadtextcolor}"><b>Last Post</b></font></a> $sortarrow[lastpost]</smallfont></td>
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If you need different widths, just change the ones I put in