David Henderson , Jul 04, 2013; 05:33 a.m.
Can't answer your question directly but you're going to lose 17% of an already small screen and of course it wouldn't work in Live View-which I for one use extensively. Of course if you used Live View habitually it might be easier to hinge a mask over the LCD screen or even just learn where the 10x8 proportions fall on the gridded view afforded by Live View. There are six squares in the Live View grid on the horizontal axis, which equates exactly to the crop you want to make. If you simply composed as if the left or right hand column wasn't going to be there, you'll get exactly what you want without doing a thing.