WordPress: Filter the_content on Blog Template

functions.php

add_action( 'get_header', 'shireman_template_check' );
/**
 * Filter the_content on Blog Page Template
 *
 */
function shireman_template_check() {
	if ( is_page_template( 'page_blog.php' ) )
	add_filter( 'the_content', 'shireman_filter_blog_content' );
}

function shireman_filter_blog_content( $content ) {

	$length = 500;
	if(strlen($content)<$length+10) return $content; //don't cut if too short

	$break_pos = strpos($content, ' ', $length); //find next space after desired length
	$visible = substr($content, 0, $break_pos);
	return balanceTags($visible) . '&nbsp;<a class="more-link" href="' . get_permalink() . '">[more&#x2026;]</a>';
}