Rename portfolio-archive.php
to front-page.php
and change the post type for that page with a filter on pre_get_posts
:
add_filter( 'pre_get_posts', 'wpse_99860_portfolio_on_front' );
function wpse_99860_portfolio_on_front( $query )
{
// not the main loop
if ( ! $query->is_main_query() )
return $query;
// not the front page
if ( ! $query->is_front_page() )
return $query;
$query->set( 'post_type', array ( 'portfolio' ) );
return $query;
}