WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I've used silicone to attach those same caps (Panasonic TSHA) to the top of my chasis (aluminium) with no problems, and it's been running well for quite some time. And to top it off my circuit is also a 6922 followed by a 6AS7G, weird...