Thanks for all the great info to implement this sidebox - but I really didn't want another sidebox header sooo - I took the $content section as built with the Authorize.net code, and placed it in the bottom of the tpl_shopping_cart.php file (saved in an override folder). It worked perfectly and sets the verified seal with the cart info, which seems appropriate.
I copied the $content code (as required to show the seal) and placed it just above the final:
$content .= '</div>';
in the tpl_shopping_cart.php