Okay, I resolved this in such a simple fashion that I'm embarrassed that I didn't think of it sooner. Each record has a type number, i.e. 1 for header, 2 for detail etc.

SELECT RRE,
  ClaimantID,
   RowType,
   TextData
FROM ( SELECT
      RRE,
      ClaimantID,
      RowType,
                CASE (RowType)
                    WHEN 1 THEN SUBSTRING(TextData,1,132)
                    WHEN 2 THEN SUBSTRING(TextData,1,180)
                    WHEN 3 THEN SUBSTRING(TextData,1,18)
                    WHEN 4 THEN SUBSTRING(TextData,1,32)
                END AS TextData
    FROM dbo.TMP_ClaimValidationFinalTextData   
   ) AS Y
ORDER BY ClaimantID, RowType
;