[MrFixr55]

I think that the cap is there to deal with varying internal resistance of the battery. I am not sure, but I can tell you from experience that if the cap is not there, there is motorboating, oscillation, noise, etc. The cap is (or should be) on the B+ side of the OPT, not the Plate side. Bias is provided by a resistor between the - terminal of the B Battery and chassis ground. At the junction of this resistor and the negative terminal of the B Battery is considered "C-". The grid leak resistor for the output tube connects to this point, providing the required grid bias. This is similar to many pre-WWII radios where the cathode of the Audio output (and usually all other amplifier tubes connects to chassis

I think that you can get away with the 22 uF 160WV cap. Since this is pure DC, there is no current draw after original charging of the cap upon turn-on.