Do not, repeat, DO NOT cut away any of your rear fork, nor any part of the frame. By dimple they mean to dent in the fork slightly where the ring "gear" would rub. You need a further 1/4 inch of clearance which is a lot and that big a dent could weaken the fork at the dent. I would suggest either getting a different bike with adequate clearance or you could attempt to widen the fork to clear. The idea is (more or less) to spread the fork slightly, then return the fork to parallel, but 1/4" wider. Spread the forks slightly, then bend it back parallel around a block or spacer of the correct size placed at the point where you want it wider. Heating the fork (but not excessively) with a propane torch at the points you want it to bend makes bending it much easier. Do it outdoors and with a friend standing by with a fire extinguisher. The paint will burn, so plan on repainting it. It is sort of tricksey doing it the first time. It would help to practice a few bends on an old junk bike. Gently and slowly are the way to go. Remember, you only need a very slight bend. Don't over do it, or bend it back and forth in trial and error or you will work harden and crack the metal. If you are careful and work slowly and methodically you can do it without heat.