At 39 kilo's you would be best advised to bring the weight forward onto the pillion area (in front or over the rear uprights). Not only will you place enormous strain on the rear sub frame, but the bike will ride front wheel light. The real problem is stiffening the sub frame without making it too rigid or the vibration will tear the frame just fore or aft of any strengthening point.
If you are dead set at carrying so much weight, brace from the footrests up like so...
