Between indicator mounts is 9 inches - if its of any interest to you I tried to fit a soft tail 'tri bar' for a soft tail that I picked up on ebay, because I was told it would fit with a simple modification but on trying to fit it - it was nowhere near it......

The light shape itself was very close if not right - it was the shape of the  mounting bracket that were in the wrong position and the bolt size eg., where mounted to indicator - guard rail points

the bracket is plastic with brass ferrels for bolt holes they were the different size and thread and position