I converted a 95 or 96 M400 at one point, two of the chainrings were riveted together, and the crank spider was tiny. I had to use a dremel to cut them apart. I can't remember what I did to tighten the single ring down. You may be in the same situation. I got lucky with a magic ratio at 34x18.
Unfortunately that bike was stolen within a month of conversion so I can't say how sturdy that ring would be without the other one for support.