Well im no expert but as far as i know you can probably get away with some not too aggressive 35"s? Maybe at least.
You would need to make sure they don't rub on the turns. Are you wanting to go for monster mudders or just nice all-terrains?
The way the tires will look will ultimately depend on how big a rim you are wanting.
But otherwise, Nice Truck
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2006 Ford Ranger 3.0L V6 4x2
1985 Chevrolet El Camino 5.0 V8
1965 Ford Falcon Station-wagon 302 V8 (current project.... still)