I'd run as wide a tire as your rules allow. The more rubber you can get on the road, er, dirt, the faster you'll corner. Faster cornering = faster straightaway speeds.
If the outside of the fender is a factor, then beat it out or cut it out as your rules allow.
Concerning the inner side, are you restricted to a stock rim or can you run an aftermarket? Are you allowed different offsets? How about wheel spacers? I ran a stock based class that switched mid-year to 8" slicks on 7" rims. I had a bad time with tires rubbing against my rear frame rails. A pair of 1/4" wheel spacers made all the difference. Your need may vary, as may your rules.
Those who live by the sword get shot by those who don't.