If the mileage matters you may need to make some changes...
Top speed will be better than with a 235, but not much. Those 6.1 gears even with the tall tires mean the motor is spinning pretty fast. Stock the 350 is probably good for 500 to 1000 more RPM than the 235 without strain, but will you want to cruise down the highway at 4,000 RPM? And I'd bet at that speed your mileage will be terrible at best.
I have a car with a 350 and an overdrive. In OD it gets 24 to 26 on the highway running 70 and turning about 2100 RPM. Direct drops the mileage to about 17 and the engine speed to about 3000.
While the car does not directly compare to the truck (effective final drive in OD is 2.0 to 1) I'm using it to illustrate the impact on fuel mileage turning the motor faster at the same speed.
Turning the motor 4000 or so to drive a 5000 pound big box through the air at speed is going to use a lot of fuel! You might want to measure how tall the tires are and then use one of the online RPM calculators to determine how fast it will need to turn to achive your desired speed.
Then consider your options to reduce the speed of the motor once you have achived that speed. It will never get great mileage by today's standards, but I'm sure it can be improved.
