The ZED Leviathan would be my top choice for a 6 channel. Otherwise I would go with M5's suggestion, as it would grant you more flexibility in choices. Plus if one amp dies, your not totally ampless.
Just curious, are you running a subwoofer amp also?
I had to get creative on my last car as I could only fit so many amps in the trunk, so the tweeter amp ended up under the passenger seat.