Of course, its also possible my 'light clunk' is emanating from somewhere other than the head stock, since I had the wheel off and brakes apart. Will go back and re-check everything - the clunk only comes after a hard jam on the brakes and then a quick release.
The brake arm pivot was rusted and seized into the arm for the caliper, I had to drive that out with a punch and tried to clean it up as best I could but I wonder if that maybe where its coming from, too. I have an extra one here, but, does anyone know if those arms are unique to the 350/360/400 - or were they shared with bigger bore SOHC bikes?