Given that there is no way of measuring the extra forces that are added if you weld directly to the crossmember, or even the strength and metallurgical quality of the steel used in the crossmember, I would just pay the money and buy a stock, bolt on tow bar. Not worth the risk and legal liability if it goes wrong at 65 mph. Cheap insurance.

Remember it is not simply the 400 lb tow ball weight that is the issue, but the vibration, torsional forces, and lateral forces involved.

Have a good look at the wall thickness of tube they use in the bolt on commercial tow bars, and then have a look at the wall thickness of the crossmember. Now add to the torsional forces acting on that crossmember due to the 6" drop in it caused by several hundred pound being vibrated fore and aft. Now realise that this crossmember keeps the ass end of your truck together......get the drift?????