You didn't mention what exposure time you consider to be too short, but is 10-15 seconds about what you are seeing? Is the bulb designed for 120v and being run at 240v?

Of course, it is possible that you want a minute of exposure time in order to dodge precisely - maybe an alternative to the ND filters is to make a 20" print with all the adjustments you want and then to re-photograph it, making a more or less straight print of the copy-neg.