For those not in the US, Ford was badly burned with the 1999 Cobra Mustang. This car had nothing to do with the Cobra we all know, just a name applied to a Mustang pony car as it is a special edition.
Anyhow, the motor was the 4V 4.6 DOHC engine that was rated here in the US at 315 hp. Problem was,when people started getting them, testing them, tracking and dynoing the cars, it was clear the motor made nothing like 315hp. Somewhere around 280 was more like it. Ford issued a recall to fix the cars involving a free flow exhaust and some other stuff (check on the web, or, here, I'm sure some member had one of these cars).
Since then, at least with SVT (Special Vechicle Team) cars which involves the Cobra Mustang and Lightning Truck, they were underated slightly.
A NA 4.6L DOHC motor typically puts out around 290-300 at the wheel, at least the ones I've seen at our local dyno shop. So, that should be fine in a mild RF GT40 and offer good performance and drivability.
The Cobra Mustang engine I'm referring to as being underated is the one in the 2003 Cobras. It is a smaller but more advanced version of my truck motor as it is a supercharged 4.6L DOHC engine running 12lbs of boost. Rated at 390hp and about the same torque, it consistently on dynos lays down around 390-410 REAR wheel hp and about 430 REAR wheel torque figures stock. My truck motor is 5.4L and SOHC, 2 valve heads, with much longer stroke. Lots of torque but doesn't want much RPM. Still, the truck hauls ass and I've left many a sports car owner for dead at the lights, just don't ask me to turn much!