EFI will help gas mileage but a 4.0l doesn't really "sip" fuel, lol, you would be around the 16mpg range.
2.8l, 2.9l and 4.0l all have the same transmission to block bolt pattern.
You will need pretty much a whole donor vehicle since you need to swap over all the engine compartment wiring, and fuel system.
Good articles here on engine swaps:
http://www.therangerstation.com/tech_library/index-engine-fuel.shtml
Since you are after better MPG and more power, which generally don't go together, lol, a 2.3l Turbo swap would be something to consider.
4cyl turbos get the better 4cyl MPG when you keep your foot off the pedal, but have that extra power when you want it.
Or throw caution to the wind and do a V8 swap
These all entail about the same amount of time, 2.3l or V8 would require different transmissions from current 2.8l trans