Re: 4.6 sohc engine swap to a p71..questions?
Have you considered a 5.4? They are easily modified for a substantial boost in torque and HP, and they can fit under a stock hood with an aftermarket low profile intake manifold. Stock engine mounts work and the stock dual exhaust bolts up with minor mods to the "H" pipe.
Keep in mind what you are building the car for. Are you are building a weekend "toy" or is this swap for a daily driver "sleeper"? These are heavy cars and a 4.6 just doesn't have the torque (torque is what accelerates the car, not HP) to move off the line without going to rear gearing that will destroy anything close to decent gas mileage. A "warmed over" 5.4 will run 14's all day long, be reliable, and get mileage in the teens for everyday driving without pegging redline in every gear.
Don't forget to make sure your trans is up to handling the extra power. Softer accumulator springs and drilling out the separator plate (up to .100, depending on year) for increased fluid flow will result in longer trans life and quicker shifts without the hard (bang!) shifts usually associated with higher line pressure. Again, these mods are for an everyday driver. Don't forget brakes and suspension. A P71 or Sport is a good platform to start on and just the thing for showing Euro machinery drivers the blue oval on your trunk.