Can anyone please explain the differences between these two engines? Is everything the same except for the tuning (carbs, cams, etc...)?? Also, will a XL600 ('85 or '87) fit into an '84 XR350 frame? My XR350 engine is in need of a "freshening" and I have an opportunity of purchasing either an '85 and/or an '87 XL600 engine which also needs rebuild. I was thinking that if the large bore engine will fit into my frame then I may as well add some CC's to my ride. Your thoughts.......???