Woa what you said is confusing :-p
SELECT rank FROM stats_individual WHERE name=$post[field5]
Will not work in the post biut

You may want to try putting in the showthread.php
PHP Code:
$ranking = $DB_site->query_first("SELECT rank FROM stats_individual WHERE name='$post[field5]'");
That should select the field rank where the name is whatever
I am still bit confused on what you want, but try that.
Also then in the postbit
put
Rank: $ranking[rank]