It's hard for an engine like this to be 2 things at once. A low-rpm performer needs to keep the mass of air in the intake low so the speed is high and the air can accelerate quickly into the cylinder. At high rpms, you want the paths all large so a lot of air can pass through. The pushrod 4.0 tends toward the low-rpm motor. I'm not saying you can't make it better, but Ford developed it from the drawing board for what it is. It's a lot of cubes in a small package and it delivers good torque and power for compact trucks and SUVs. It's 200ft# at 1,000rpm keeps up with about any modern engine of its size because the 4.0 was tuned to be able to do that. You start grinding out ports and increasing cam duration and you will have a motor that might make more power higher up, but will not return the money you through into it and will definitely not make 200ft# at 1,000rpm anymore.
For the high-performance version, look at the SOHC 4.0. It has a variable intake manifold so it can do the low and high speed stuff, overhead cams to get the valve stems and pushrods out of the way of the ports and get rid the the rpm-limiting valvetrain. If you want a bolt on 50hp, swap in a SOHC 4.0. It's got your performance heads.
The next step is 4-valves per and variable cam timing which pushes the power up another 50+ hp.
Whatever you can do to your pushrod motor you will definitely feel in the seat-of-the-pants because your wallet will be lighter.