Things to consider:-
Are the existing beams already loaded? Whats their condition?
Connection from new beam to existing. I'd be more inclined to go with a bolted connection with web stiffeners welded into the existing section.
If it really is going to be just a chain thrown over a steel, I'd expect stiffeners to be fitted to the new beam as well to prevent buckling. The strong points on a plane are usually located at the nose and under the wings (to be confirmed). Wouldn't jacking it up be a better option than suspending it and risking damage?