Dave, your statement "From what I understand, the issue is all wagons have SLS" indicates you did not read my above post, stating that my 1979 wagon parts car does not have SLS.. and I only found that out when another person said their wagon did not have it....
Further, there is a possibility , given the above, that the SLS was designed to bolt onto some of the basic frame which the wagon already had... Since we are talking the same body style.. and like the situation where the clutch pedal mounting is included on automatice trans vehicles... to reduce the number of different mold, etc... to reduce cost....
Many springs are available for all the MB's ... and I know for sure that TWO DIFFERENT heavy duty springs are available for the rear of the wagons.... one described as " for countries with poor roads".... they are color coded from the factory....
So , while no one may have posted about changing over, or has posted but did not fulfill the necessary physical criteria, I believe that a suitable changeover is certainly possible with a little information and care in choosing components...