really kind of over kill and risky going with a ball joint on your control horns, if your worried about hardware, then junk the push rods for the rudder and elevator and go with real 256 rods there stiffer than the supplied H-9 ones,that will flex and bend under stress.
the rods for the ailerons are fine as there only 4"long and really are not going to flex at all, but the rudder and elevator rods are a bit too flexable being over 6"long IMO,so I replaced mine on my 2 H-9 warbirds.
the horns provided by H-9 are typically fine for a 60 size plane.
adding a ball joint to your horn is just adding a week spot,unless you get the type Bruce suggested, if you use a typical clevis and secure it with fuel tubing like any other model in that size range you will be fine.
if you drip some thin CA in the holes you drill in the control surfaces, thin CA will harden the area,just let it dry then redrill the holes,after you have lined up the back plate, it will make the area tougher a tip I got from the warbirds section that works well to harden the rudder, elevator horn locations.
note the scale tail wheel location on the Mustang,a good place for a ball and socket joint,I would steer away from using one on a control horn,unless it's one of the ones specific for control surfaces,and replaces the horn all together,but it's really not necessary.