I have a set of "341" casting forks; they came off a CB550, and have the dual mounts. The OEM CB550 fender fits, and so does a single-brace fender from a 750 (don't know what year/model)...
I also have a *long* fork lower, which is a single disk setup, and has the same caliper/fender mounts as the stock CB500/550, but is about 1 1/2" longer between the mounts and the dust seal/wiper...I don't know what this one is from (eBay purchase)...
I also have a CB750 front disk; it is ~3/4" bigger than a CB550 front disk, but will bolt up to the same wheel.