Well, no sooner did I finally settle and make the cut than I walked back in the house and remembered what you said about the radius not having to be centered to the fender, like the blue car you pictured. I do think it would look better had I cut more off the front than back, and I can move the axle and inch or two, but oh well. If I fudge it any it'll be forward. Last year I ran 14x6s with the tires inside the fenders but it wasn't very tough looking with the narrow track to say the least. So the plan now is to let it all hang out, like the 10s on there now.
Cut yourself some 18 gauge that looks like this, with the exact opening you like and weld it on. You could have both sides done before the sun comes up if you had the stuff and put your mind to it.
another thought is to find another fender with a good opening and weld it to yours. might even be able to bring it out some to cover the tire.