Many thanks for that !
In desperation I actually tried measuring a photograph of a pair of CB550 forks with a tape measure across the screen of my phone.
Then basing my calculations on the sliders being 352mm long I got an overall length of between 716mm to 727mm based on two different photos.
When you get the chance, could you possibly manage to check your fork slider length.
The sliders on my CB750F1 measure 352mm from the axle centre line to the top face hidden under the dust seal.
I'm hoping that the 550 sliders are the same length.