Serious Help

I have a 92 Chevy short wide. The prob is, I just droped it gettin ready to bag it and the tires I have are F****** up my front fenders. I am plannin on runnin 20's. I dropped it 4 in the front and 5 in the back, but I am about to have to raise the front back up an inch or so. The question is what size tires would I need to put on a 15 inch rim to equal the size of a 20? And what size that won't screw up my fenders when I turn. Help would be greatly appreciated. Thanks.