I have written to a number of W42/W43 owners about deterioration of the I beam. It was put there so the hull could be used as a cutter or a ketch, since they have different main mast locations.
If the I beam is so deteriorated as to no longer give a good support to the mast step under the cabin sole, then it is only necessary to install a steel post going from the base of the mast down to the top of the ballast, and doing away with the portion of the I beam in that particular location. This of course requires temporarily removing the stainless post in the salon, as well as the remnants of the I beam in that location. The new subfloor post should have a plate welded top and bottom to spread the load on the top of the ballast, as well as to the visible post in the salon. This subfloor post could also be secured to the closest bulkhead with bolts.
If the post is made about 1/2" short of the required length, then a 1/2" spacer plate can be installed when the cabintop is jacked up to reinstall the salon portion of the post. These two sections of post should be bolted together, with the spacer between them.