I trimmed a little over an inch off mine when moving up to 35's.
And a sharp box cutter worked best for me too. Have extra blades handy.
I first tried a dremel, a multi-tool, and a jigsaw, but the heat generated was too much, and did not work so good.
I used a home made compass/jig with a marker to trace a line following the edge of the flares for a stock appearance. Cut the line carefully with multiple passes of the blade.
Mine didn't turn out so bad I think. I ended up cutting off the front fenders later for flat fenders. LOL!