Do you have, or can you make a frame jig?
Are you a certified welder, or know someone who is?
Do you have a large MIG or TIG welder, or have access to one?
If you answered "No" to any of those questions (which you most likely did based on your question), forget about cutting the frames. The steering necks are the highest stressed part of a motorcycle frame. Weak welds will fail. The alignment of the neck to the frame is critical. 0.5º out of alignment can cause unpredictable or even dangerous handling.
All of those are the technical reasons to not do what you are thinking. Then there is the legal aspect, which is beyond my scope. Your local government or law enforcement agencies can likely explain how what you are considering is likely criminal. Possibly even at a federal level.
Bottom line: Forget about swapping necks.