Interesting. I kinda figured 15 mph would be about best case….even with the improved gearing.
Going from memory, but I think Cosmo used to report 20ish mpg with his highly modified setup. (Dual carb 261 and an OD?)
Wind resistance and fuel distribution are not going to favor a 235-powered AD truck. A guy can probably make some marginal improvements on the fuel distribution. Not much of anything on the aerodynamics.