You can put 2 22 mfd 450 v. caps in series , connecting the (-) of the first to the + of the second, to make an 11 mfd 900 volt cap. You also need to put a resistor across each cap (100K if i remember correctly) for voltage equalization.
Also the exposed part of the can of the first cap will be "hot" so it should not touch anything. We did this in an RCA PA amp with 807 tubes which had very high voltage electrolytics and it has worked great.
Here is an article about it:
http://www.nostalgiaair.org/referenc...d/rsd0944b.htm