Here is the part http://icsbb.com/web_e/html/02_products/show.aspx?kind1=12&kind2=13&num=92

And I would like to remove everything above the red line

 

I have no experience with MP5's but based on the picture and the link, you may have to remove the front hanguard and slide that piece off, unless of course you want to saw or dremel that piece off somehow.

 

Yeah but when you slide that piece off there is a big gap