Something to check on that era of Ford 4.6 is if it's one with weak spark plug threads. Some penny pincher decided that only 4 threads in aluminum heads would be sufficient. Uh, no. There's been many instances of those engines blowing plugs out.
Fortunately there are fixes. Skip the Helicoil for this. The good fixes drill out the plug holes and thread them for a metal insert with more threads and a new spark plug seat.
Trick Flow Specialties makes Twisted Wedge heads for the 2-valve 4.6.
Trick Flow Cylinder Head Upgrade For 2 Valve 4.6L Ford Modular Engines - Hot Rod Magazine
The big difference from stock is the TFS intake valves are angled toward the center of the engine while the exhausts are left at the same angle as the stock heads. The TFS heads are designed to directly replace the stock heads without changing anything else.
The TFS heads can use the stock Ford cams and followers, gaining efficiency through optimized intake and exhaust flow. (I dunno what a stock ECU would do with these heads, leaving everything but the heads, valves and springs stock.)
With the "modular" engines you have to know which plant they were built in, because unlike previous Ford engines there are significant detail differences depending on which plant built the engine. I dunno why, it's as stupid as the wimpy spark plug threads.