my two cents

a A series with good comp eg 9:1
good tuning eg EFI with a long runner manifold
spend some time on the head to make the power
and dont put a cam in

cam shaft selection is very important any cam that makes the engine idel rough will cost eccon

a nice set of pipes and

then if it dont have enough power use a toyota blower with a micro switch at 3/4 throttle

or you could take the $3000 and buy a ecconomic car drive it and have a fun little datto for the weekends

valves and seats shouldn't be a problem unless you live somewhere that they are just phasing out leaded gas.

LPG can be a good choice, however, to gain the pull potential from this fuel, it will be necessary to build an engine that reaps the benefit by design.
The simplest things to be aware of are the fact that a higher compression ratio than we are accustomed to will be desirable in order to release the full potential from every combustion. Something like 11 to 1 will suit LPG quite nicely, but this would rule out dual fuel operation.
Another is the fact that it is a 'dry' fuel & it offers no protection to valves or valve seats, so it will be necessary to ensure that both the exhaust valves, & the valve seats are compatible with LPG. No big deal, but something to be aware of at build up time.

You don't necessarily get more mileage from a litre of LPG, but the significant thing is that it costs so much less to buy that litre, so the cost per mile/kilometer IS significantly less.
Another upside to LPG, one that B210sleeper has already mentioned, is that oil changes are all but a thing of the past. LPG is also a very CLEAN fuel & oil contamination from the gas is almost non existant.

Those who reap the greatest benefit from it are those who amass big mileage, like Taxi's & commercial travelers. It's these people who allow their operational savings to pay for the LPG installation, then go on to bank the savings for every mile they drive.

convert to lpg ;) if you're in AU it's only $200 out of your pocket i hear. it's much cheaper and you save a lot ( yes big savings) on oil changes.

add a turbo, more efficiency if you behave...

efi is worth every penny. just to give you an idea i had a stock 12a turbo making 125 at the treads... did some mods changed the computer from a microtech d4(analogue) to a better one (wolf 2d).... doubled the power but kept the same fuel economy... up to 240 at the treads at 6km/l average... a good computer tuned properly works very well

Efficiency improvements help...

Straight out lies generally work.

You can't have both. A carefully driven, off-boost turbo engine is good on the fuel. But when under boost, fuel usage can triple or more.

Fuel Injection

I agree with Dodgeman, driving style is a major contributor to economy, but so too is avoiding twin 45's.
My a15 (1608cc) runs twin 45's and is FAR from economical. Good power though.

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.

does things like balancing, port designs, valve sizes help? im only used to adding on to motors which in all cases has made things worse but more power. would using a gx head be better than stock a12/A14/A15 head? i gather 1200cc would be a good start but what other things could help? efi is good but i have found it less reliable compared to a simple carby and fuel pump (if u know what i mean) but does it make much difference in petrol usage? i suppose there are so many options with a series it does make it hard to choose what u want? i want to make my 1000 ute into a car i can take long distances (eg camping) but with good fuel consumption and great reliablilty.