Custom Post Templates

I think this approach will work.

1.create template for single post like singlepost.php( default single post template),singlepost-99.php,singlepost-101.php.

2.now put just this code in single.php

<?php
global $post;
get_template_part('singlepost',$post->ID);
?>

what this code does check for single post template for current post by post id if not found call singlepost.php.

Important Link:

get_template_part()