Ideas on heatsink needed?

I finished the transformer for my dc hid ballast but I don't have a heatsink yet for the transistors. Does anyone know how big of a heatsink I would need for some 3055's at about 50 watts of power. They are TO-3 case.

A 2N3055 has a thermal resistance of 1.5 deg C per W and a maximum junction temperature of 200 deg C. At 50 W the semiconductor will be around 75 deg C hotter than the heatsink. That would put the maximum heatsink temperature of around 125 deg C. If there is a maximum air temperature of 50 deg C, then the heat sink can be 75 deg C hotter than the air. To dissipate 50 W with temperature difference of 75 deg C, you need a heatsink with a thermal resistance of 1.5 deg C per W.