I dug and dug, looking for info on XLR600/650 hybrid engines, found your thread on TT, followed over here, and no new news! Get on it man!
I think I stumbled on the original thread on TT, because I was searching for info on using a 650L head on an XL600R.... I know bassackwards. But I was just keeping my options open, since my frame/suspension is already modified, so carb placement isn't an issue. Going to follow your lead and start a thread over here, seeing more interest.
Good luck with your build, I'm following.