Anyone running 315s on 18" wheels?

We would have no idea what would fit on your car. You will have to do your own measuring (always! the best answer)
We don't know how much you tubbed it, narrowed the rear or not, moved the springs, ladders and coilovers, you didn't even tell us what body style you're talking about.

Just for reference.... I am running a 315/65, but on a15x12 rim. And I have a full tub, a four link, and enough room for a 18x31 et street. (18" wide section not an 18" rim).