Quante-One more thing you should do. Take a look at the way the Associated TC5 mounts that lower arm. It is almost bullet proof. The little amount of flex those short vertical plastic inner hinge pin mounts protect the pin from bending and the arm from bending. This is also apparent on the associated pan car upper arm mount. That flexible plastic caster block protects the upper arm and pin. We hardly ever break the upper arm. I have tried to make something similar on mine. It is somewhat succesful but the base of my arm is not long enough. In your schematic your arm looks like it has a long enough base.
John