Originally Posted by cross-country
If you used a 450sx(RFS 04-07) motor, you would not need to chop the frame. You would only need to move (cut off and reweld) the rear motor mounts and find a yz400/426 carb bell. If you find an SMR it would already have a 6 speed in it. There is one on KTMtalk for sale.
It's actually the bottom mounts that you need to move/reweld, and you also have to modify the front mounts, either by milling down the cases on each side or by bending the mounts out, IIRC it's 1mm per side.
But you probably knew this seeing how you built one.