http://www.wowhififever.com/RCA_mi9377/pic_3.gif
The above link shows an RCA amplifier with the same network yours has, with different component values. Only difference is in the above URL the network is connected to the centre tap of the output transformer (B+), rather than to ground. From the perspective of an AC signal, these points are at the same potential.
Consider that the peak to peak AC voltage on the primary of the output transformer is what will determine the voltage rating of the capacitor, NOT the value of the B+ supply. That's why I would go for some nice big 1.5KV capacitors or even 2kV just to err on the side of safety.
RCA MI series amps are beautiful equipment, I wish all electronics was built to that standard!