97 should be the 7.3. Its no powerhouse but it works, expect no better than 15mpg when towing, you may squeak 18 unloaded. Diesel is a solid .75cents hire per gallon here than gasoline, but thats recent. Usually its a full dollar higher. Fuel mileage is so close it wont matter when you get into the expense of the larger brakes, 12qt oil changes and 6 E load range tires.
A 1/2t truck to a 1-1/4t truck is a big step up, do you really need that much truck?