1966 Spark plug replacement

[old timer]

What E3 spark plug is used in 1966 Thunderbird with 390 engine

 

[cougarsunlimited]

What E3 spark plug is used in 1966 Thunderbird with 390 engine

The E3 website doesn't show a plug for your car. Why??? Their cross-reference for the BF42 recommends a E3.72 plug.
Personally, I wouldn't bother with gimmick plugs. Stick with the Motorcraft BF42 (SP420) or Autolite 45.
The double platinum Autolite APP45 plugs should last longer.

A Ford engine engineer told me the best type was a double iridium plug because it has the sharpest point for the spark to emit from.
Iridium lasts longer than platinum plugs so that's what Ford was installing in newer engines.
Unfortunately, I haven't found any iridium 18mm plugs that fit, so the double platinum is the best choice.

 

[74 Harley]

I've run Autolite 45s forever. Never had plug issues.