Having a 75 with acros and clams, I can say two 175W bulbs will do. However, if I could do it again I would go with 250 HQIs. Also, I have 2 3' VHO actinics FWIW.
To give you a straight answer, I say 2 250W HQI 10K with 2 actinic VHO supplements. If you have the money and the space, do 2 250 HQI 65K with 4 actinic VHOs.