I found that using a Holley carb from a 85 Mustang GT, that the idle circuit is revised to give you a 14.7 a/f at idle.
It was idling so lean, in pulling plugs to check them, that they were so clean that I could not find evidence they had even been run.
The issue with that became that the primary tubes on the headers were glowing red. I would think that condition would mean regular steel tubing headers would at least need to be ceramic coated to help them survive that kind of heat at idle?
That series of Holley carbs is called 4180's.
No matter what you do to those carbs, you cannot change the a/f ratio of 14.7 at idle. You can only increase the amount of fuel at idle which by design won't change th a/f idle ratio.
I had revised the fuel bowls and other emissions plumbing on those carbs to a 4150 configuration so that they looked like a 1850.
The reason that I mention this here, is that if anyone is having issues with something like a 4779 750cfm dp Holley and they need to lean the idle but can't, any of the 4180 series carbs are a useful tool to experiment with. They are all going to be 600cfm carbs and the ones with truck applications are just as good to work with as the Mustang GT carb and less expensive to find.
As far as spark plug heat range goes, if we are talking about original Ford iron 4v heads, either closed chamber or even open chamber, the 4v plug should be an AF32 (or equivalent) but it is not uncommon to run one step hotter to an AF42 to help on keeping the plugs cleaner.
In the Ford heat range system, a 25 is a racing plug, too cold for street traffic driving and will foul up within 60 seconds or so of idling on a cold engine. It is way too cold for street driving.
I even found them too cold for Autocross, which I did regularly 30 years ago. Even the 35 needs a WOT burst occasionally. For Autocross I actually found that the 45 was better.
These numbers are how I cross reference spark plug heat ranges.
When you switch to other brands you will find that the other brands do not index heat wise exactly the same and can be 1/2 step higher or lower then the Motorcrafts.
The ONLY heat ranges that accurately cross over to the Motorcrafts are the Champions. NGK's are close but do not have as wide of a range so are not exactly the same.
I hope any or some of these details helps. I'm not here to criticize, just share what I have learned over the years.
Incidentally, the P-E engine controller, which replaces the Motorcraft ignition brain, does retard the ignition at starting.