the most likely reason for the smoke was lack of drive to the HOT, you should be able to get a DMM reading on the grid of the hot, think it should be around -70v (neg to the chassis, positive to the control grid of the 6JS6).
My guess would be a bad horz osc or some cap in that circuit. The main thing to do is to monitor the HOT cathode current, if it goes over 260ma then you have a problem, no need to try anyhing else until that is under control.
Yes the caps have some over voltage in the ratings, but they are 45 years old so no need to test that rating
Just go slow, read read read, get some old manuals off the net.