Horsepower with mileage options

A lot will depend on the type of driving this is intendend for. A lot of idling time and stop and go is different than improving mpg on the county roads and interstates. Some will also depend on the climate. Since emissions are not a condideration, carb and timing is fine.

I think the 318 is very decent base. In stock form, using a timing curve from 67 or 68 non-CAP, should pull 21-22 mpg highway with the current drivetrain and provide good pep.
Reducing weight is always a plus for both goals. (feather duster for factory example amed at milage)
A four barrel carb will help power in the rpm range. As others have said it should not hurt mpg. The extra two barrels allow it to operate with more efficency over a wider rpm range. If trying to improve 1/4 mile times and mph - this is what is needed. Horsepower is all in the top rpm - its not the same as pep.
Small tube headers will help the upper rpm and possible the entire range. These will need to be custom for what you want to do.

Changing intake to aftermarket is sometimes better for mid and top power (2500 - 6000 rpm) and often worse for idle to mid. A lot has to do with fuel distribution during part throttle. With good vacuum this is (part thrtottle/low throttle distribution) less of a problem but it still can be worse than many stock intakes. Losing weight is good of course.
Aluminum heads. Losing weight is good. Others have already commented on other impacts. This change will require some time working on part throttle timing and fueling.