Actually, there’s no problem with the position of previous/next, it’s depended on the wordpress core function.
If you like, you can edit single-portfolio.php, find the following code
<?php previous_post_link('<strong class="prev">'.__('« Previous Post').' <br />%link</strong>'); ?>
<?php next_post_link('<strong class="next">'.__('Next Post »').'<br />%link</strong>'); ?>
Just change the position to each other.