With proper modification and a tire that's still legal to drive on the highway, 54". With stock suspension you may be able to move the tire size up a few inches. Maybe about 5" added to stock tire size. Also keep in mind that different size tires require a change in speedometer gears. Ask a local parts store about the speedometer gears, and if you don't change them and put bigger tires on, you'll be reading a lower speed than what you're actually doing, which would be inconvenient when you think you're going the limit and get pulled over for speeding. The same goes for changes in differential gearing. If you want an all out mud truck that's as hardcore as you'd want and/or could get away with on the street, you'd want about a 30" lift with custom made leaf spring suspension, beefed up chassis, Rockwell 2.5 ton front and rear 4x4 axles, 54" Super Swampers, a big block of any kind or possibly a small block built for torque, low slung bumpers to comply with laws to do with bumper height, possibly headlights mounted on the lower bumper to comply with headlight laws depending on your state, beefy transmission of any kind so long as it handles the abuse, and a roll bar and brush guard with some fog lights. However, you most likely wouldn't be up for that type of truck. So add a couple inches, change your gears, and you should be fine.
