Hi Jim-
I've never seen another E39 so I only have my boat as a reference point. My guess is that you have the one piece molded sole throughout the saloon and galley. And no way to get in there to do anything. We had the same and chopped out the whole thing so we could gain access to the 40+ years of filth, replace rusted steel beams, water tank, wiring, everything BUT replace the step which looks pretty good.
Here is the sole with removable panels, soon to be clad with a pretty flooring material.
Underneath originally was just a continuous space. The big blocks of lead in the keel are directly under that and that is what the various carbon steel components were attached to. I've heard there is an actual steel beam in there as well, but our boat just had large cubes of lead with bolts threaded in to hold the aft steel athwartship beam that ties the aft bulkhead together. Our athwartship steel beam was in pretty bad shape, so we glassed in a G10 beam to replace it. The original glass that was over the lead pieces had actually delaminated off the lead, so we removed it all, dried out the putrid water that had gathered down next to the lead, refoamed the blocks back in place and glassed the whole mass back together. It sounds like more work that it actually was.
Here is the new G10 beam, I'm hoping for another 40 years out of it:
In order to have a real sole, we needed some athwartship support for removable panels. Here are two of them:
I bring all this up for two reasons:
1. I think to properly fix your step, you are going to need to make some room to work. Like removing a bit of your sole (probably soul too.)
2. I am still not exactly sure how our mast step is tied structurally into the boat, but when I was working on the sole, I was thinking that I could probably box in the step and tie into our athwartship sole supports. I decided to just leave well enough alone and moved on to the many many other projects that I am working on.
If I had to make a new step, I would first want to get out the old one and see how the heck that is tied in. I'd probably build a new mast step out of G10, mostly because it is a material I can work with without having to hire someone and it is perfect for an occasionally wet environment. And it is stronger than all get out.
I know this didn't answer your question directly, but hopefully it gives you a peek into what is under your feet. Just keep asking questions, there are others much more knowledgeable than I that have answers as well.
Regards-
Chris