I thought we had a posting for this subject but having done a search I can see the question comes up a few times but with no single source of the answer so.... here goes....
The manual says you should fit Motorcraft HG2 (Resistive) plugs gapped to 0.025 inch which is 0.635 mm
If you look at some spark plug comparison tables like
https://www.sparkplug-crossreference.co ... RFT_PN/HG2 you will see there are lots of choices.
Most owners use NGK equivalents but go for a slightly colder plug like the DR9EA for "normal" use.
https://www.ngk.com/heat-range-conversion-chart-2
Make sure you get the R version and not the D9EA version as the resistor helps prevent high boost pushing the electrode up the body as Willy found out the hard way a few times.
Don't go buying them from a motorsport shop, get them from a motorcycle shop or Ebay as they are much cheaper.
You need to unscrew the nipple from your old plug (or buy a bag of new ones) as they do not come with the nipple, just the thread!
Fitting them is a pain as the "well" they live in is very very narrow and a standard spark plug tool will not fit down, you have to machine one down very very gently as Paul did for me in his lunch break in the Dagenham tool room
Final thing I wanted to mention was that I have always found it very easy to kill a set of plugs. Starting the car a few times as I move it around the garage over the months without a proper run seems to ruin them. They always need to get up to full temperature which means going for a blast to prevent this happening. Maybe my car runs a bit rich, not sure but I am not the first to say this.