Quoted from Biju:
Is there concern in going significantly over on voltage? There's a local place that has 15,000uF 63v capacitors in stock and I seem to remember this should be fine although the memories of my classes from the 80s have been eroded by hops and grapes.
Edit: my research sounds like I technically can do this but at the expense of overall lifetime of the capacitor.
Voltage rating of a an electrolytic capacitor is a maximum rating so it is OK to increase the voltage rating, within reason. I think there is a decent reason not to run a 400v electrolytic cap on a 5v supply.. Generally I go for 2x the working voltage. 15,000uF 63v would be OK, it will likely be physically quite large thought and not mount well.