Thanks a lot Borber,
I did my dc current measurements at the 215 volt supply. For Output voltage 400 p-p at the secondary of the transformer, i measured the DC current drawn is around 113 mA. So the power dissipation comes out to be 6 watts/ transistor.
I might sometime use 1000 volts p-p output, for that case the Power dissipated/transistor wld be approx. 10 watts.
I have also starting using BUH51 instead of 2SC5392 which have gone obsolete. THe BUH51s have Pd = 50 watts @ 25 degree C.
Do i still need the heat sinks, as the actual power dissipation is below the limit? Thanks again for your help.