Well, upgrading to a pushrod 4.0 is only good for 10-15hp. But the torque goes up by 50 or so ft#. It feels like a totally different engine at truck revs--it's like having a V8. In the quarter mile, there won't be much difference.
Getting your hands on a cammer 4.0 will net you a good 70hp.
These motors are all pretty maxed out. What I mean is they are going to detonate if you try to press much more torque out of them. Torque is cylinder pressure and more cylinder pressure is going to mean the fuel goes unstable and detonates before you want it to start burning. That means you need rpm to make power and that torque can't be increased much. The SOHC (cammer) 4.0 makes much more power than the 4.0 or 2.9 because it can spin faster at the same torque.
You can't squeeze more out of your 2.9. You are better off swapping it for a pushrod 4.0 and enjoying the huge boost in torque. But it won't be fast. If you want to go fast, the 4.0 SOHC has great heads on it. Extremely straight shots into and out of the cylinders and a good round port section. These motors ar 12 years old so they are obtainable and they will bolt to your tranny. Your $1,500 toward a 2.9 will be completely wasted. Best to find a real motor to work on.
