Get a shutter with a scale for the 305. The 210 will be almost exactly one stop faster than the 305 at equal aperture needle settings. So f/11 on the 305 scale will be f/8 on the 210, f/16 will be f/11, etc.
270 doesn't work as well, so... just don't bother carrying a 270
Use the 210 component on the rear and the 305 component on the front, you'll get about a 255mm combination.