1) Preview with no individual frames selected
2) Select as much of the interframe film base as you can with as little of the frames either side as you can. Select "Lock Exposure" then preview again
3) Now select "Lock Film Base Colour" and preview again (not sure if this extra preview is required)
4) Now select "Lock Image Colour" (I not sure if you need to do this)
If you now select and "Scan" the frame you want you'll find the histogram shows black and white triangles for each of the RGB channels, and these can be adjusted to avoid clipping.
"Save" the file then convert to greyscale in Photoshop and apply levels to taste. Soem go to the trouble of just working on which of the green or blue channels look sharpest or give the nicest greyscale (I think).