which head for the 4.6L?

[Hammer]

As stang00 said, port and polished PI heads can expect around a 40 rwhp increase.

From the factory FMS heads alone will get you around 35 rwhp, add the FMS intake, and you're looking around 45-50 rwhp. These are all stock numbers from the factory, so any additional work done SHOULD give you an even better increase....

The worked over PI heads give you an extra 5-10 rwhp at peak, but the FMS heads give a better power output along the curve, lower compression ratios, and room to grow if need be.

Part #s:
M-6049-D46 : heads
M-6067-D46 : Head changing kit
M-9444-D46 : Intake install kit
M-9424-E46 : Intake Manifold

Possibly a new Engine Management Computer (M-12650-A461 Auto trans. or M-12650-B461 Manual trans.) or custom\dyno tuned chip with the stock computer.

I would contact Ford Racing (586-468-1356) or another knowledgable company such as Dallas Mustang (800-MUSTANG) for specifics on your application and help with purchasing the exact parts you may need..