Into the weeds engine design for fuel efficiency discussion.

You also got to factor different engine sizes with similar performance the smaller one needs more rpm and overall gearing probably negating any mpg advantage. Plus why do people generally want diesels most will say torque, but really cause it makes peak power at a very low 2500 rpm which allows you to operate at peak power at normal driving speeds for long periods of time so you can put everything the engines got to tow/work. Most truck gas engines are gonna make peak above 4000 rpm so generally out of range and wouldn't handle being at peak for long periods of time. The only main way to improve NA power idle to 2500 + rpm is displacement bigger the engine will generally have more towing power, above 3000 rpm don't matter much for towing but good for fun even then you probably want to keep peak hp no higher than 4500-5000 rpm. To me that means you either build the biggest you can or the size that will make your desired hp between 4500-5000 and build it the best you can to be good on gas or at least don't do any thing that will kill mpg. It's easier to ruin mpg then gain it, to me it comes down to cam choice for best mileage and head choice for best power, look at LS and Gen 3 Hemi they make power with heads and smallish cams.