This discussion has been interesting but increasing fuel economy is very difficult.
There are only two things to do to reduce fuel consumption:
1) reduce the work you are asking the engine to do
2) make the eninge more efficient

So all of the ideas to reduce the work of the engine; lighter wheels, reduced aero drag, manual trans or lock-up TQ are good ideas and will result in improved fuel consumption no matter what is done to the engine.

The real problem is that actual fuel economy may not be effected even if the engine is more "efficient". This is because most of the time the engine is operated in a part throttle mode. It is very difficult to improve part throttle fuel economy because as the engine is made more "efficient" the throttle position changes (closes) which causes the engine to overcome more pumping work (the work the engine needs to overcome to suck air past a partially closed throttle plate). It's one of those things that you take 3 steps forward with say compression ratio and 2.5 steps beck due to an increase in the pumping work, so the 10% efficieny improvement becomes 2% or less and this is just not seen when driving the car.

Idle fuel consumption is pretty much based on engine speed with fuel/air ratio also a part, so the lower the idle RPM and the leaner the lower the fuel consumption will be.

Engine-wise I suggest keeping it simple; use a high compression ratio for thermal efficiency and a low overlap cam for good idle at a low RPM with a lean idle carb setting, with this you might be able to see an improvement with fuel economy.
Any other thing you can do to reduce the engines work should also show an improvement in fuel economy.