That looks great!
I've really gotten into the 20" AR's. I like them a whole lot more than the carbines. As a matter of fact, I only have one M4 carbine now. I sold the rest and purchased more 20" A2's!
Some will call me crazy but hey, I like what I like and I shoot them better. I know, it's a personal thing but it's what works for me.
I'd like to do a built like yours! It looks great! It's so retro it's cool!