M-Machine do a complete inner & outer sill sub-assembly complete with jacking point and it is a very reasonable price.
I fitted on to a 1964 850 shell a while back and it went in so easily compared with doing the job with 3 pieces - inner, outer and jacking point. The whole job of cutting out the very rusty old sills, cleaning back, positioning the new sub-ass'y and tack welding took about 5 hours. Then it was just final welding and linishing back.