The weakest part is the part you threaded into… My humble opinion, a M8 grade 8.8 stud, used on, as previously mentioned, drilled out, and re-threatened original bolt hole , using the head of the left over bolt to be threaded as well should be sufficient (and the weakest link before the grade 8 bolt snaps) … and use the original rubber swaybar bushings versus the Polyurethane bushings which are not very forgiving!!
If you wanted to be really strong, you have to cut the crossmember open or make a plate sitting on top of the crossmember and weld it on.
would be a nice project…
thinking out loud:
drill a hole where the bolt supposed to be the size of the bolt head to be used, stick the bolted in the hole welded in place then make a plate with two holes where the threats only protrude and weld that plate onto the crossmember.
there are many other ways to do it and make it stronger…