[morzh]

I think 10uF is fine for 8uF as the first cap. It will step up the B+ a bit nut not too much.
The second cap does not matter much if it s more than 4uF. Could be 10uF also.
The 1st cap if you want to be perfectly within the voltage specs could be made closer to 8uF by paralleling smaller values or serializing two 16uF caps (many say you should also equalize them but I simply use two 450V one and then you are fine or so I think).