This is an area to use "more than needed" because we are not too concerned with a pound or two extra. Also do not use a brittle grade of steel. Brittle steel (or anything) does NOT belong on cars...or bridges. You will get FAR MORE strength per pound by the SHAPE of the piece rather than the thickness of the material. ie. gussets, bosses, trusses, ribs. Common sense makes good engineering also.
In order to solve the equation with a solid number result, you would need all the knowns; weights, torque, safety factor, engineering drawings, material specs, weld strengths..freebody diagrams...ughh...I am going to go with the "Make it extra strong!" solution to save the legwork. A good CAD program will do it but you still need DATA.
A photo of the existing setup would help to guesstimate.