which head for the 4.6L?

[Hammer]

The 99 PI heads\intake swap gives a good power increase for a decent amount of miney. It is also not a direct "bolt-on" and minor changes to your setup have to be made. Bad news is that compression is increased and its not to friendly if planning on going forced induction.

The Ford Motorsport Cylinder heads flow better (more horsepower) and compression ratios are much more favorable if you go to forced induction (blower). Bad news is they're quite expensive...

when you said that they have the best flow numbers (stock) you meant without being ported etc. right?

Correct

can you buy aftermarket heads that are already ported or anything?

You can buy the 99+ PI heads and port them. I've also heard of some folks even porting the FMS pieces....

In my opinion, if money is no object, the route to go is the FMS heads\intake combo. It's almost a straight "bolt-on" affair, gives great power numbers, and is ready for a blower when you are.

My old 98 with that setup made 385 hp at the rear wheels with about 6.5 lbs. of boost present at the intake (8 lb. pulley)from the blower. This was with a conservative tune keeping the timing down.
Over 400 rwhp would have easily been attainable with a more aggressive tune and a 10 lb. pulley.