Require help with 301 redirect

A while back, I moved my blog from a folder (manukhanna.com/blog) on my domain to the root (manukhanna.com). To redirect visitors, I entered the following code in the 404.php file of my theme:

<?php
header("HTTP/1.1 301 Moved Permanently");
header("Location: http://manukhanna.com/index.php");
exit();
?>

The problems I am facing are:
I have t5wo issues regarding this:

1. This redirect code comes into play if the requested link is not found, and the visitor is directed to the home page. I actually want that the reader be redirected only if the requested URL has manukhanna.com/blog/* pattern. Else my custom 404 page should display.

2. As the redirect is in the theme file, every time I change my theme, I have to paste this code in the new 404.php file. I know I can put the code in .htaccess but don’t know how to?

Much appreciate.