Is this the right way to mount a 2N3055 or any TO-3 form on a heat-sink?
TO-3 devices are typically mounted on the inside of any enclosing heat sink, rather than the outside. The way you have mounted it would prevent the part from being soldered directly onto a PCB.

Also, there are TO-3 specific heat sinks available, many of which would take less PCB real estate and typically provide better cooling than the one you have used. Heat sink selection involves required space, material (copper / aluminium / other), shape (flat / enclosing / raised), fin type, and expected thermal wattage to be cooled, as parameters.

Wrap-around design heat sinks also shield nearby components from the heat generated by your device, an added benefit.