A harder working motor tends to be more efficient, yes, up until you're hitting ~75% load/throttle, after which it can get a bit more inefficient since manufacturers seem to run a bit richer to safely get more power/torque.
In terms of gearing, IMO it's better to go w/ taller if you aren't looking to break the bank in terms of power output since you can always just drop a gear to get more at some speed. The 7.3L diesel makes more power at a
lower rpm, ~175hp@1800rpm becauseit's turbocharged, and more or less sits at 200hp the rest of the way through. Your smaller engine makes more power but also has a much greater range of operating speeds. Another advantage diesels have is more energy per gallon, ~10-15%, which means a gasser getting 10mpg is equivalent to a diesel getting 11-11.5mpg all things being equal. They can also be more efficient at peak, but this is only a few percent difference. The biggest difference comes from offload engine efficiency, for instance VW's TDI engine from the early 90s is more efficient off-load, for instance at 25-50% torque/power at some engine speed, than the Prius' engine. This is another advantage for diesels, since they can be geared shorter w/ less of a drop in efficiency. Since people looking at efficiency tend to buy diesels, manufacturers tend to give gassers gearing that maximizes power w/o looking as much at efficiency anyway. For the most part, gassers gain way more from gearing changes at lower cruising speeds, say 75mph and below, than diesels do, although both can see gains.
To get some specific insight, look at what your max and min payload/grade/speed are in order to figure out what kind of gearing range you would need to maximize efficiency, lets say you need to be at ~50-70%+ torque at that speed/load, and look at the rear end/transmission options you would need to do that. Going with taller gearing may not be a problem since you could just use 4th instead of 5th, etc... when towing and you'll have the same power output at the same speed. The only disadvantage is that you wouldn't have as much pulling power in 1st, so if ya need to go up large grades towing a few tons at ~10mph it could be a problem, and you may not be able to tow as much up a tall grade going as fast, for instance you probably couldn't tow two tons going 100mph up a 3-5% grade or whatever the figures are, as well as go as fast unloaded in fifth. Otoh, you should be able to see 20+ average mpg easily since you'll have a much taller fifth that'll allow you to have the engine make more torque/operate more efficiently when unloaded, going downhill, etc...
In terms of aero, the
best boat tails make great toppers for pickups if designed well. If you're driving w/ little or no load, it's in it's normal spot and you see better aero, and if you need to put more in it or are towing then you can prop it up in the normal topper shape, and if you need to haul something really tall just take the top off. It'd probably be fairly easy to do something w/ square tubing and sheet metal if concerned about cost.
Edit- I should add that at 5000lbs total weight even my little pickup can make enough power, ~50hp, to tow an unloaded four horse trailer up a ~1.5% grade in 3rd w/o getting near peak power (sticky needle), since it has a close ratio 4MT. Tossing in a taller rear end along w/ a 5MT would allow me to do the same, except I'd end up in second and I should be able to go a bit faster doing it, while getting ~40-45+mpg unloaded in fifth. It's all about gearing compared to power output over the range of speed/weight/grades you'll see.