The legs have to be long enough so that when the fender bolts to the clamps it clears the tire. I wanted to maintain a dust boot, so turned the leg "round" in a lathe just below the boot to pinch the clamps.


I'm sorry I can't tell you when Honda switched to longer fork legs, the forks on this bike are 404's from a 77 CB550K model. They are 13+9/16" from the top of the axle to the top of the leg. On my 750K3 the legs are 12+3/16", not long enough.
I'm using an 18" rim which gives me lots of clearance, but tried a 19" wheel for hymodyne and it just cleared but probably should be spaced. For me the fender looks a tad small on 19" but it should work.
I wish I knew how to post these pictures smaller.
When I planned this bike build this is the recipe I came up with to use the fender I liked, I'm sure there are better minds that can make it work differently.