I really do no t think you will see a 23 % decline in fuel mileage. Some yes, but you may get better fuel mileage when towing. Nothing worse then lugging an engine for fuel mileage.My 47dodge had a 3.73 rear. I changed to a 4.10 rear fuel mileage when towing went up about 1, and not towing did not hurt it any. I think the big cam pushed the happy rpm range of the engine up, and running were it is happy makes a lot of differance. Do also remember that it takes "X" amount of power to move the car no matter the gear ratio. So the only losses can from engine losses of spinning it faster.

Now having sais all that, I do agree with others you should try it stock first and see how it does. My 300 has the 2.88 rear and I have towed boats up to about 3000lbs with no issue. I would change to the 3.06 in that car if I was towing a lot with it. So you are were i would have changed to.

Change the rear end oil, and I would use synthetic oil. Must say I have to wonder about towing all the time, it is a car, and not designed for it. A very good car, but still pushing it.Expect to do more maintenance, and do drive it reasonabily This will go a long way to keeping it in one piece. I do tow light loads with my 240, but have a truck for real towing.

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1977 240D turbo