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.
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