Is MattoRX7 & Matto1kUte the same person?

OK, lets start with the basics.
Is this ute intended to carry a full load of wood for the fire 3 times a week & a couple of loads of bricks on odd numbered days? If so, then the long stroke A15 will haul the load much more efficiently than the A10 ever will.
Perhaps it's just you & your honey out for long, slow Sunday afternoon cruises along the beachfront & fuel costs an arm & a leg. Then a nicely tuned round port 1200 will be a good choice.

Do you want to just sit & smoke up the tyres all afternoon, then a blown A14 could be the answer.

But it looks like you want spirited performance, but with economy to boot. On the upside, the B20 ute is the lightest of them all, so a 1200 engine will very likely be the best compromise. I say a 1200 because it will fit your chassis without mods, while the 1974 & later A12, will not.

If you want economy & performance, then which one is more important to you? For now, I will go with economy in a high density urban environment. In this case, a round port 1000 head will serve nicely as this puts the heater fitting in the right place. See if you can get some 1000 pistons as these have flat tops & raise the compression. A little more compression will improve the efficiency of each drop of fuel burned.

Induction.
There are many to chose from, but for the moment I will rule out the most efficient one, EFI, on cost grounds. You now have two basic choices. The original manifold with a larger Weber carb, or an aftermarket manifold with two SU carbs.
I will ignore the dual Weber side draught setup as the last thing you are likely to get with this is economy.
A single sidedraught Weber is a better choice from both a cost, as well as an overall performance perspective in my view, but neither of these two inductions offer ported vacuum for the distributor, & this will negatively affect economy.

The single downdraught Weber is the simplest, but correctly tuning it can be a fussy affair. None the less, it will surprise you just how well it can run, ... & how economical, it can be.

I rather like the twin SU setup, but the manifold & parts can be a little difficult to source. As long as the carbs are in 'as new' mechanical condition, & tuned correctly, they will be a source of absolute joy.

Ignition.
Electronic. There is not really any other choice & a high energy system, like the Datrats one will light the fire in your cylinders with a high degree of certainty & reliability. If every droplet of fuel is burned up efficiently on every combustion stroke, then high efficiency is assured, & the big fat spark from an electronic ignition is the best way to do it.

Cam.
This is a little more difficult as most people tend to over-cam their engines. Something with a 'light sports' profile, that idles smoothly at about 700 to 800 prm will be fine. It doesn't go rump rump at idle, but will pull well & run strongly out to about 6,000.

Exhaust.
With a stock inlet manifold, my personal choice is a 1000 coupe exhaust manifold. This still supplies heat to the inlet manifold, & has a dual outlet. Extractors do not supply heat to the inlet manifold & this allows a condition where not all of the fuel flowing in is converted to a gas, & remember, it's gas that burns, not the liquid fuel that finds its way into the cylinders, washing the oil from the walls, then geting into the crankcase, diluting the oil.
Cold air induction is fine for engines operating at, or near, full throttle, but at economic cruise, you need heat.

With twins, an extractor will be the go, but keep the pipe sizes within reasonable limits. In either case, an exhaust pipe of about 2-1/4 should prove adequate.

That should be enough to get you thinking. Anything that makes more power, does so at the cost of more fuel consumption as you need to burn fuel to make power.
An engine like this should prove a good starting point for reasoned debate.

Now we could go on with wheel size, tyre size, gear ratios etc, but the most important fuel saving thing in the car is the driver. If you know how to drive, you can save a bundle.