sorry to disillusion you, but a 650 motor wont fit in the 350/500 frame.
the 650/750 is a much bigger casting and bigger bike weighing in at a massive 400 lbs or so. the 350/500 's were about 350 lbs.. I had no idea modern technology weighed so much
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A wise man learns from his mistakes..
A genius learns from other peoples mistakes.
Thats what I thought.
The top of the motor probaly wont clear the top of the frame and the down tube might need to be modified.
got to find a boneville or tr6 in my price range.
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My bikes not fast, Nor is it pretty
But it sure does wake up the neighbors.