This is seemed better to have a rigid joint with two spherical ball races. This also implies some sort of housing for the bearings. This means adding some sort of hanger for the muscle end at each joint. It seems best to machine all these parts. Then the fasteners get more complicated.
Cost goes up and up.
This is not the way. A little reality check is in order. This should be a simple, consumer type of machine. Welding is O.K., but it should be the type of welding that can be done by a robot. Two rod ends give a good knee joint.