6 bubs may be enough for your SPS but only the long run will tell. Possibly someone with more experience will chime in.
Hell, 4 might even be enough considering your tank is only 17" tall.
If I had those fixtures...
Instead of paint inside, I'd look into these
Sunblaze reflectors, or might try to make
these ones work. Be cheaper.
Of course, I'd have at least 3 timers. Maybe even 6. That way there would be a more natural light up and down photoperiod.
But I always go overboard.
Regardless of the canopy you build; you may need a fan in there if it's wood, or at least louvers if it's metal.
Sure will be cheaper annualy over your MH