If I can't find one used that I can get my hands on faster, I'm going with Brown Dog Designs, but I want a harness of similar design to ones like this, that he can comfortably run in, pulling. Not a weight pull harness.
BDD Walking Running Harnesses
That design with the vertical chest strap, which leave the shoulders and legs free to move, is what we need.
He has a 20" neck and a 33" chest.