This should work for the postbit template, you should be able to do similar with postbit_legacy if that is what you are using.
Find:
HTML Code:
<vb:if condition="$post['isfirstshown']">
{vb:raw ad_location.ad_showthread_firstpost_start}
{vb:raw ad_location.thread_first_post_content}
</vb:if>
<vb:if condition="$post['islastshown']">
{vb:raw ad_location.thread_last_post_content}
</vb:if>
<div class="content<vb:if condition="$show['first_ad'] OR $show['last_ad']"> hasad</vb:if>">
Change to:
HTML Code:
<vb:if condition="$post['islastshown']">
{vb:raw ad_location.thread_last_post_content}
</vb:if>
<div class="content<vb:if condition="$show['last_ad']"> hasad</vb:if>">
<vb:if condition="$post['isfirstshown']">
{vb:raw ad_location.ad_showthread_firstpost_start}
{vb:raw ad_location.thread_first_post_content}
</vb:if>
Adjust padding around ad as needed.