Your XB engine will be a 2V. The Phase 3 GTHO carby is a 780 cfm vacuum secondary carb. They are still about (the genuine HO ones are rare) and occasionally for sale. They were a good unit, easy to work on and responded well to changes. If you buy a well used 780 though, it may be worn out i.e. throttle shafts, etc. I have been told by some engine builders that a new 750 vac sec does a fine job as a replacement.
The cam grinds for the HO's are available from some grinders as well.