A D800 would absolutely be much better. Having said that, many D3 and D700 images have been used that big. One does not generally walk up and closely examine photos in such a usage. From 50 feet away they could look fine, if properly handled by the graphics firm. If you get the job, though, and it pays reasonably well, I'd absolutely rent a D800 or D800E. The problem would then be that you'd be hooked -- there is a difference!