http://www.ohio.edu/people/postr/bapix/SB610.htmThis fellow dealt with the same problem.
His approach is the same as my thoughts, that you need to use several diodes in series to replace each selenium diode. a 1n5408 has a PIV of 1000 volts. He used four diodes, with a guessed safety margin.
Check and consider replacing your filter caps and equalizing resistors.