I called and spoke with an engineer a couple of years ago when I went to the extreme beefing up my 7' rotary cutter. I have a 100 hp gearbox on the cutter but the sides were getting killed when I mowed some of the rough areas on my property. I'd make a required sharp turn and the long deck would swing into a rock or a tree and the blades would start hitting it. Anyway, I built a full cage around the existing deck with 2X2 square tubing with 3/8" sidewall. I attached the tubing using 2X3 angle that is 3/8" thick. By the time I got finished, my 7' cutter ended up weighing over 1500 pounds.
One thing I didn't touch was the two weak pieces of flatbar that go from the top of the 3pt hitch to the rear of the mower. The engineer told me that they are basically only there to allow the 3pt hitch to pick up the entire mower; not to lend any support. He even advised me to take them off and replace them with chain if I have a lot of areas where I go down a steep hill and then right back up another hill. He told me that if I braced those bars so they wouldn't bend I'd take a chance of breaking the top link to my 3pt hitch or worse. Basically, those thin bars are designed to give and flex to prevent damage. If they get too ugly, just take them off and replace them with lengths of chain.