I'd be tempted to use a floating trap detail template to handle something like this.
I'd expect that you'll need to place every ~ from your sample line in your trap, and set the width of each field to something much larger than you think you'll normally require, like 150 or 200 characters.
If you can zip a copy of the report, or a representative portion of the report, and sent it to me, I'll have a look.
Also keep in mind that Monarch "guesses" at the characteristics of database fields by scanning only the first 250 records of the database. So if record 251 has a field that is much wider than it is in the first 250 records, you'll have to adjust the field width manually. The best approach is to use the Field List dialog for this.