I would do it. You can get additonal spread simply by raising the lights. On my 60" 120-gallon, I have only 2 MH (30" each). They are roughly 18" above the water's surface. The tradeoff is intensity for:
1. Greater spread.
2. Lower electricity bills and fewer bulb replacements.
3. Ease of access. You can build the canopy in such a way that you can reach into the tank and do maintenance with the lights on.
Here are some pictures I took today of my tank. There aren't any noticeable dark areas except where the plating corals are creating shade.
