Those wheels look good! I hope you've narrowed that rearend and are adding tubs inside the bed. Forgive me if you've already done this, or said you are... it's been a while since you've updated.
I have 295's on the back of my fleetside on 10" wide wheels and there's clearance. A 305 might fit, but a 315 would be TIGHT. I know there's more room on a fleetside than a stepside too, but I don't have any idea how different they are. I just know I've read stepsides are hard to get 10" wheels in the fenders.
I'm sure you've done your homework though. It'll look killer when you're done.