[Big Dave]

Considering that you are talking 100% city driving the only thing you could do for a radical MPG improvement is seriouslt radical surgery: Remove the engine and put in a 4BT3.9 Cummins diesel. The diesel is far more efficient in low-power operations as you don't modulate power output by making the engine less efficient (throttling) but by merely reducing fuel flow.

This is a mechanically ambitious project and definitely not for the faint of heart.

No, I don't know if the Cummins will fit. Fords have notoriously small engine compartments. I do believe some motorheads have down 4BT3.9s into F-150s though.

Cheap-charley efforts will yield cheap-charley results.