You are going to get a million different answers to this. Most will revolve around what your budget is. If it were me, and if your current engine runs well enough, try and find an explorer/mountaineer engine complete from a salvage yard. They will have GT40/GT40P heads that are much better than your stock heads. They will also have the GT40 intake and 65mm throttle body. That alone will give you a nice improvement in power. You can rebuild that engine while you are still driving the car. Once the engine is finished, you can swap engines in a weekend. There is a salvage yard in my area that has two explorer 5.0 engines complete for 250/300. Depending on how much work you can do yourself, you could do a budget rebuild on one of these for less than a thousand.
