Not speaking from experience here, but sometimes the 2.8 lives forever, and sometimes explodes randomly. If you aren't afraid of swapping in a 3.4 if it does explode, than I would pick one up. IMO you're starting way ahead if you find a 4.0, but I would take the 2.5 any day over the 2.8.
totally agree..ive had a XJ with a v6 and it ran good for a yard vehicle and wide open assualts on the driveway for a whole summer with no exhaust..