Does anyone know if xs750 special fork tubes are the same diameter as xs750 standard. I just want to change the fork tubes assembly not the triple tree.

Pretty sure the only differences between the two models are aesthetic. You should be fine, the 750 special just has a bracket on each fork to support the fairing. I have one of them Vetter fairings from my old '79 still if you're interested.

Actually I just looked up to make sure, but in 78 they came out with the 750 SE, and it then had longer fork tubes than any 750 previously. As long as the ones you have are post-1978.