The De Tomaso years at Benelli were a bit strange. De Tamaso's philosophy was "you can't steal from a theif" as justification for wholesale copying of the honda cb500/550 engines. Although I have never tried, I have heard from many reputable sources that the benelli motor has the same mounts as a cb500/550 and will easily slot into a 500/550/650 frame; I have also heard that they are not an exact fit and may require welding/cutting. I have seen people interchange the benelli engine covers on 550 engines since they are finned and way cooler, and I know benelli 500 quattro owners routinely use honda parts to fix their bikes. Unfortunatly the chassis aren't really the same, neither is the bodywork, only the running gear.
the 750 SEI is basically a 500 four with two cylinders added on, the don't even try to hide it. I don't think this would be any more or less difficult than putting a 500 or 550 in a 750 frame.
