 350-400 rwhp
350-400 rwhp is tough to do with a NA 4.6. Assuming you want this much power all the time (and not just when the nitrous bottle is full/on), forced induction would be my suggestion.
A supercharger can get you there for about $4-6000 (depending on your choices and whether or not you can do the labor yourself.
Going the engine internals and heads route (which is where all the stuff about heads and cams comes in) will cost about the same amount to do it right (heads, cams, springs, retainers, intake, TB, forged internals, machine work, gaskets, gears, etc, etc), and will come up short of your power goal.
You didn't indicate how old the engine is.
A high mileage engine is likely to feel the strain from boost or just adding parts on the top end (at some point you reach the weak link, which might well be the rods or nearly worn out bearings/rings, etc.).
If you're engine already needs an overhaul, that would decide me toward a forged internals build plus heads and cams.
In that case, be advised that "stage 1/2/3" cams or heads are simply the 3 off-the-shelf options aftermarket suppliers often sell. What they actually do differs wildly, so you have to do some homework looking at the options.
For example, the larger Comp Cams (sometimes called "Stage 3) require new springs, retainers, and a custom tune, plus properly matched gearing. They would also not be a great choice on top of a weak bottom end, where they will put a lot of stess on the old OE parts. This cam also has a rough idle, and might present driveability issues in a daily driver (and this is a subjective topic as well - some people love the rough idle and don't mind the associated issues).
The 270 cams (stage 2) have a lopey idle, don't require as much extra work, but don't deliver as much maximum power at high rpms. More streetable, though.
Heads from such companies as FoxLake (my personal favorite), Patriot or Livernois are NOT true aftermarket heads such as exist for the 5.0 family, but are re-worked 4.6 OE heads. I'd visit the various sites and do some reading as to their relative merits and demerits.
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