Put the stock filter box back in, and buy a drop-in filter that fits it. K&N or one of the newer dry washables.
Getting more power from that early 4.6L won't be easy or cheap. Its intake system is perfectly fine for what it is, but you can't get much more from it. When the engine goes, look for an '01-up engine with the PI (Performance Improved) heads. It'll take some rewiring to make it work RIGHT, but it's worth the effort, IMO. For the best stock performance, look for a Marauder engine, as complete as you can get.
