Here is a site (the only one I know of about Challengers) that may be of use to you www.namotu.net/
. I don't have blue water experience of the boats, but understand they are more than capable in that environment.
I'll be pulling my mast in a few months to refurbish it, I'll know more about the step then. In the meantime all I can offer is that you examine the collar the mast sits in to get an idea of what may be required.
There is a great article on exactly this subject on Sailnet that should be of assistance to you.
All the best