here's a chart that will help you figure out what kind of lighting you'll need.
PAR vs Distance, T5, T12, PC - Updated Charts
Running 2 bulbs per side isn't going to be a penetration issue as much as not having an even spread across the width of the tank. That can be remedied by placing the fixture higher up if you have enough extra light intensity to compensate with.
A good even coverage and decent PAR are most important. And even if you don't manage to get even coverage and have dark spots, you'll just have issues with the parts that are shaded.
6700k is the 'daylight' area for bulbs, although different manufacturers have different ideas of what that means, so sometimes one will look different than others. I like the ~6000k lights, they have a slight warmth to them, while 10000ks have an operating room feel to them. A lot of people will mix the two to get a nice balance of light.
I wouldn't cheap out on the fixture for a large tank like that, spend the extra and get a quality fixture with good reflectors and bulbs. It would look a little weird if you don't have a canopy covering it, but you could use 1 4ft and 1 3ft fixture to get a better fit over the top of the tank.