I couldn't tow my 26' boat using the rear bumper! The boat was actually pulling the metal part and causing it to deform! I opted to install an ARB tow master towing hitch!
Remember to ask for grade 8 bolts. For mine I found bolts with hexagonal allen heads.
Since the 2 lower holes doesn't have a thread I suggest you to add a (inside the cross member) reinforcing steel plate as wide as possible, with the 2 holes pattern pre-drilled and if you can, weld the nuts on it, before inserting the plate. This will help you to install the bolts with only one wrench. Don't forget to torque the bolts to at least 80 lbs and you should use one flat and one pressure washer for each of the 4 bolts.
It's a cheap and easy way to reinforce the cross member and limit somewhat it's flexing if you pull a trailer with some tongue weight.