When I first started recapping antique radios, I tried really hard to use the same voltage caps, but it turns out that not only can you not find them that low, it doesn't matter a bit. As long as the physical size will fit - and the new ones are generally smaller than the old paper foil ones - and the voltage rating is no lower than the original, you're good to go. I've put a lot of 630v caps into radios to replace 250v and 400v originals, just because I got them cheap.