That clearance can change depending on your ride height and hitting bumps or low spots when turning, like driveways & such.
5/16" doesn't sound bad depending an how you're checking it.
Closest point should be bottom corner of fender in front of the tire.
That's actually easy to trim back at a 45-degree angle with tin snips and will barely bother the paint if you're careful.
I've done that on a couple of Dusters and early B-bodies.