How should I position my fingers for this chord?

I don't know the tune, so I'm not quite clear on the question. If you need to mute the strings you show open, I'd play it 2-3-4, and mute the other three strings with the index finger. If you're supposed to have the strings open, but the trouble is you're muting them, there's a lot of ways to finger it... 1-2-3 should work, or even using the thumb to barre the bass strings.

The thumb will probably only work out well if that's the way it was originally done, because it will be very hard to switch to other chords from there.

correction:

-----x-----
-----x-----
-----x-----
-----3-----
-----3-----
-----3-----

First of all, thanks for the help! When you say play the strings 2-3-4, do you mean using the 2nd finger, 3rd finger, and the pinky?

Yep, that's what I meant. Middle finger on the 6th string, ring on the 5th, pinky on the 4th, index finger dampens the rest.

id barre the 3 strings with my index finger/ring finger depending on where im coming from and where i am going next.
this way, i could mute the others with the rest of my finger, and
have the rest of my hand ready for whatever comes next.

NoteBoat: Is that how you always play "5" chords in drop D? I find that incredibly cumbersome, compared to a simple half barre.

I've seen a lot of songs that were just moving the barre to make power chords like that, but I've always fingered them using the index. "Ironman" for example, that's extremely annoying to use 2-3-4 on those slides.

Well, I don't know the tune (didn't even know it was in dropped D, but that makes sense).

Most of the time, if I run into a chord like that I'd use 2-3-4, yes. I think there's two reasons it's very comfortable for me - first, I play a decent amount of 'inside' chords when I do standards - and the three fingers together is how I'd do this:

x
5
5
5
3
x

Moving them over a couple strings doesn't give me any trouble at all, and I just tried it on a classical guitar neck.

Second, I'm not real big user of powerchords in the type of music I play. The only tune I can think of in my playlist where I use them a lot is "You Really Got Me".

Honestly, I'm not sure that the tune is in dropped D, that's just a shape that comes up in dropped D all the time.
x
5
5
5
3
x

I find that very interesting. I would usually play that with the bent back 3rd finger, however...

3
5
5
5
3
x

I would ALWAYS play this with the 2-3-4 to make sure I was getting a clear sound out of my 1st string.

Any feelings on the bending back of the finger?

I'll bend back the finger sometimes for the full barre A form. I can get the 1st string clean with no problems. I just use the 2-3-4 fingering for inside chords - then I'm damping the first with my little finger.

I didn't initially know what you were talking about when you mentioned "inside chords". Now I'm assuming you mean chords where you're muting (or not playing) strings on both sides of the chord?

Yes - inside chords are played on the 2-5th strings (or sometimes 2-4 with bass on 6)

cool.