Look on the "gas mileage thread" on here and you'll see the 2.5 typically gets better mileage than the 4.0. The mileage gets worse once you get out of the right fear ratio (ie. too large of tires on stock gearing).
It's an AMC six based engine, it'll live forever and a day if you keep to the maintenance schedule!
I can't confirm the timing chain stretch thing, but I was told by an engine builder that if you replace the chain you should replace the sprockets, and if you replace those you should do the whole cam and lifter set. If anything, a nice double roller chain for a 4.0 will work, be stronger, and run smoother.
In any case, it should still have a lot of life!