Ford didn't produce a 289 Windsor.
EDIT:
Actually....
The Windsor is the Ford small block V8 engine family, so yeah they actually did produce a 289 Windsor. People usually don't refer to it as a Windsor though since there wasn't an alternate 289 ci displacement engine, such as with the 351 (Cleveland and Modified, which are big blocks). Windsor just designates the Ford family of small blocks. And sorry, I know all that is irrelevant to your question. I just wanted to correct misinformation.
The plug gap for a 289 is .034
the gap should be 0.034 in or 0.86mm
gap is 0.035"
.044 inch
.035 inch
.035 inch ( according to motorcraft . com )
( .044 inch ) according to motorcraft.com
i have read at another site that it is around .044
The spark plug gap is 09.96 The spark plug gap is 09.96
Check the Vehicle Emission Control Information ( VECI ) decal located in the front of the engine compartment . It will show the type of spark plugs to use and the spark plug gap
According to one of Fords websites : For a 1966 Ford Mustang , 4.7 liter / 289 cubic inch V8 engine : ( the spark plugs are gapped at .035 inch )
spark plug gap for a 2.2l is 0.060"
.045 spark plug gap