Im not a 100% sure, I believe that you will be fine in a 55 gallon. I have a 55 gallon, but no MH yet. Just blew my money on a chiller and live sand.
Eddie
I got a crocea in a 55G with 2x 175 MH 14k and 2x 110 VHO actinic and he looks like he is pretty happy to me! The clam attached him selfwith in a few days. I placed it as high as I could under one of MH. They can move so make sure its in a spot that allowes a little movement! Good luck!