I'll second what's said above, really depends on what your goal is and how much money you want to spend. The 4.0 V6 is really an antiquated design, so dollar for dollar you will not see the horsepower returns on that platform that you would see on a 3 or 4 Valve V8 or in the V6 Mustang world, the 3.7 cyclone. But if you would be happy making 330-350 RWHP no reason not to stick with what you have. Once you start getting over that power level in the 4.0 you're looking at a built motor which jacks the cost up considerably, plus all the other supporting mods to hold that power.