Nice '57 Chevy 6400 you've got there. Its very similar to mine, a former firetruck that I turned into a flatbed dump. I'm sure my drive train setup is similar to yours, except I still have the original 283 V8. Fourth gear 'High' puts my truck at a top speed of 55-60 mph but the motors screaming. 45 mph still turns the engine fast but its within reasonable RPM. I've got 10x22.5 tubeless on mine too, these tires are not commonly found but are available at the large truck tire centers, especially shops that sell tires for agricultural use.
As far as restoration I would neutralize any rust areas and give it a good paint job. For now I would change out the fluids and give it a complete service. MOST IMPORTANTLY I would thoroughly go through the brake system and don't go cheap on any fixes. Check the steering too.
Again you've got a great 'heavy' Chevy at a good price too. Its a good working type of truck, classic in appearance and a lot of fun to drive. Check out mine in the Stovebolt Gallery.
