Yes yes YESSSSSSS
I have been thinking about a real fuel miser engine for a couple of years, & what it would take to really drive your dollar further.

In my case I started with the car & for me it would have to be a B20 [Datsun 1000 Ute] as these are the lightest of them all. The B210 is a barge by comparison.

I had also thought 'A10 engine', but my 1200 GX engine in my old B10 sedan got much better mileage than it did with the stock A10 engine, so 1200 engine it had to be, but not a later A12 as the 1200 engine is a direct bolt-in. I once got almost 55mpg [Imperial gallon] from my [heavy] 1968 Datsun 1000 four door sedan with a 1200GX engine, so the potential is really there.

My version of this 'virtual' 1200 engine used A10 flat top pistons with the cylinder head from the KB10's version of the A10 engine. The flat top pistons naturally provide more compression but the KB10's round port head is worth another half a point of compression in stock trim as well.

I also wanted to wrap the inlet manifold in copper tubing that ran hot water through it for good manifold heating & I thought that an 1-1/4" SU would be good as it is a high vacuum design carb that is almost infinately tuneable.
Warmed air would also be drawn from over the exhaust manifold. Turning liquid fuel to a gas is of paramount importance & not a single micro droplet can be allowed to escape.

An A15 flywheel is nice & heavy so should produce advantages here, but it adds some weight overall. Never mind, some sacrifices must be made.

I also thought of the 56A five speed [F5W56A] as the extra gear between 1st & 5th will also be good for economy, ... & it's a straight out bolt-in.

Switching a 3.54 to 1 ratio H150 differential into the original B20's housing is no problem either as it too is a bolt-in & final gearing can be controlled with tyre size on skinny steel wheels which are readily sourced in 13" & 14" sizes. The only downside is that the H150 is an iron diff & the original B10 & B20's H145 diffs were alloy case units. Bummer.

Further, but minor weight reduction can be achieved with the removal of the spare tyre & it's mounting gear & even the passenger seat as well as the substitution of a motorcycle battery in place of the original.
The smaller & lighter 1200 radiator should also be quite adequate in this application.
The heater was an option in the earliest of the B20 Utes so leaving it out will save some weight & still be road legal in Australia.

Such a car would not be a road burner, but man, it could make a tankfull of fuel go a very looooooong way.