Lief,

The easiest way to do this is to utilize the spectreMDL montecarlo flow.

Specifically, run your normal montecarlo iterations using spectreMDL.  SpectreMDL will then generate
an output file that contains the scalar data for each measurement performed along with some statistical
figures of merit for the montecarlo loop that was run.  It should be noted that you may have to use the
"seed" parameter to ensure that you can duplicate your montecarlo run later.

In this file you will notice the first column is named "iteration".  This iteration number represents
the montecarlo iteration that produced the row of measurement results.  From this data you can
identify which run provides your worst case performance.

Now, in either spectre or spectreMDL construct a montecarlo run using the same "seed" value if necessary, with the
"firstrun" parameter set to the worst case iteration number, and the "numruns" parameter set to "1".  
Inside this montecarlo loop add the analyses you want to run (and any measurements if you are using
spectreMDL).  This will cause the process and mismatch variation to duplicate the worst case process/mismatch
variation that you determined from your initial run.  It also will save you quite a bit of time.

Hope this helps,
-scott F.