If you stick to the 300B version, the bias voltage isn't going to go much higher than 75V. You may wonder then why there's a 450V cap there. Partially I believe that it is because we stocked that particular 47uF/450V cap, and also because we usually stocked the Panasonic ED electrolytic, which is a fantastic capacitor with good temperature tolerance and long life. If your output tube had a catastrophic failure and shorted out, that particular cathode bypass cap wouldn't mind so much.
If you put a 160V rated cap in that position, you'd probably be safe, but you'd be less protected from catastrophic failure.
It's also worth mentioning that the parafeed output transformer returns to the filament, so the sonic impact of that cap isn't a high as it would be in a regular old series feed amp.
I did manage to fit these in my Paramounts, but I didn't have other giant caps in there competing for space:
https://www.parts-express.com/jantzen-audio-47uf-400v-crosscap-capacitor--027-948You should also be aware that the hot end of where that cap attaches is also available as a terminal on the output transformer (5 or 10, can't remember which), and you may find it easier to anchor the cap on top of the OPT and just solder some wires onto the ends of the cap and connect them where they need to go.
