I'd go along with the A10 [which is NOT available in the US] only if fitted to an extremely light vehicle as the 59mm stroke is really just a bit toooooo short to be hauling a B210. For US conditions, I suggest either a 1200 engine, or A12A or possibly best of all, the 1974 A13 with its long stroke & small bore.

The sodium filled valves may be just fine in a heavy duty truck engine that can be on full throttle hauling heavy loads up long & steep mountain passes, often & for long periods of time, but as far as I can see, would contribute nothing to an economy engine. They would cost a packet too.

High swirl head is good, if Nissan made a small port one & it's readily available in the US.

Synthetic oil is probably good, & so would low tension valve springs. Jet coating would be good if going for a world record, but for a local competition, the cost is probably prohibitive.

Again, the tyres & mag rims are very good, but probably cost prohibitive, however steel rims, even if narrowed & welded with skinny radials pumped to max rated pressure [+10%] would serve well in the initial contests.

I agree with the single SU carb. They are available in a small sizing, relatively low cost, are all-but infinitely tuneable, have no accelerator pump, & because of the variable venturi design, maintain good vacuum under most driving condidions. A simple adapter would be easy enough to make & I still think that a heated air intake is a good idea.

Electronic ignition & probably platinum plugs with widened gap [to fire that lean mixture] should simply be a given. You will need all the spark you can get so as to fire the leanest mixtures possible.

The thin rods, knife edge crank, headers, alternator & ribbed belts are probably very expensive overkill at this level of competition but the heavy flywheel, likely from an A15, is both cheap & a very good inclusion. Attaching a machined plate to the clutch face would add more weight & increase the benefit.

An earlier suggestion of very high static compression ratio is actually worthy of a second look.
Maximum cylinder, or compression pressures are attained only with 100% cylinder filling, but at idle, for example, the compression pressures are actually quite low since only a small percentage of the cylinder is filled with combustable mixture. The piston can easily be half way up the bore before atmospheric pressure is attained, so effectively, the 'percieved' compression ratio can be as low as 3 to 1 at idle, particularly with a very heavy flywheel. In an economy engine, if we can get the compression pressures to match the 9, or 10 to 1 that other engines see at full throttle, but do it at very low throttle opening & low volumes of charge in the cylinders, then a benefit would be achieved.
Just don't screw the Webers onto such an engine.
For the exercise, 15 to 1 static may possibly work out to somewhere near to 8 or 9 to 1 'equivalant' at very low throttle opening.

As I see it, the problems with this project revolve around ready availability of suitable components at an affordable price & remember, the US didn't get all the goodies that we did like B20 utes & A10, or the later 1974 & onward A12 engines.

I think that with sensible selection of available US sourced components, that 65miles per US gallon can be achieved in a B210 car but it won't be easy.
Remember, for those of us who are old farts in Australia, Great Britain & Canada that's 78 miles on one Imperial gallon or 125.5k on 4.54609 litres of fuel or 26k on 1 litre. [About 3.85L/100k for metri heads]

That's a bloody long way on a milk bottle of fuel in a realatively heavy car like the B210.
How about you open a new thread in the projects forum & keep us posted.