The 6 position UTG will work. You might need a shorter buffer, but I'm not 100% sure on that.
You should join
www.m4carbine.net if you're not already on it. It has tons of info on all types of weapons, not just M4's. They are really slow at approving your membership, I think it took a week for them to approve me.
If you ever get the urge to get a gas pison AR, don't do it. I really don't like mine very much.