OK, this is Crown Vic information. But if this is the same 4.6/5.4 the P71's used:
They are the 2 valve engines. No cam phasers, but still have the mile long timing chains.
My 2000 4.6 popped a plug, #5 which seems to be a common one, 3rd one back, passenger side. Used one of the Heli-Coil Kits, (The one that Locotech2000, he's a member here, showed a video of), worked great, and could be done one the side of the road in a pinch. Only bad part was, the kit and the loctite was almost $300, but much cheaper than a head, and it came with enough to do all 8 cylinders is needed. Also had to replace the intake manifold, but that was just an early one issue. That was fixed in 01 or 02 from the factory.
My 2007 4.6 didn't pop a plug, but at about 185,000 it started eating coolant. The rest of the car was getting tired, so it was traded in on my 11 F-150. Only thing ever had to do to the engine at that point was replace a set of spark plugs. Always started, every time. Honestly, if the rest of the car was in a bit better shape, and my knees and back were too, I would have replaced it. But the truck is much easier for me to get in and out of. The newer ones don't have the plug issues, but you still have to be careful. I always use a 1/4 drive set and just make it snug....
I wouldn't worry about a later 2 valve triton. The early with the plug issues, and the 3 valve, I would not want.