In the current system, he'd probably be considered as an attacker - so it's an area with the like of RVP, Rooney, Kags, Mata, Januzzaj, etc. He'd have to displace one of Mata, Rooney and RVP. Either that, or he'd be in midfield as an upgrade on an already very good player in Herrera, or played at wingback. While his versatility is great, it's not often that you spend £60m on a player and not know where you ought to put him. I guess the big difference in our favour is that we actually know that Di Maria is versatile, whereas Ozil looked out of sorts in a lot of the games he had to be played away from #10.
It's weird how people think that the reason we aren't playing 433 is because we lack wingers. The reason we're playing 343, is so we can get our best 3 players on the pitch. It'd be silly if we went to all that trouble building a system that serves to get the best out of Mata, Rooney and RVP and then end up having to drop one of them. It'd be senseless to have spent an entire pre-season working on a system to then throw it away.
Just an aside on the six NO,10s point; signing Di Maria doesn't give us less number 10s - it arguably gives us one more. He definitely improves our quality, but you can't really think that this signing makes us a more balanced side?