For fuel economy and low-rpm oumph, together, your best bet with a 318 is increasing the cylinder pressure. But the early factory 318s are already pushing the pressure limit for todays pumpgas.
> an optimized ignition curve will pay dividends.
> a free-flowing exhaust is a must.
As to other options;
>A 4bbl will not likely affect hiway fuel economy and with a small-Primary spread-bore like a TQ, could actually better the factory 2bbl.
> the stall-speed will also up the take-off oumph, but may affect the hiway fuel-economy. It sorta depends what you are swapping out.
> Swapping the factory trans out for an A999(with a 2.74 low gear) will better the first-gear oumph, while the Lock-up feature should better fuel economy.
> IMO, for your stated usage,
an overdrive trans, together with a rear gear swap, would be the way to go; together with optimizing the advance curve.