For more or less stock GT-40 setup, the Motorcraft AWSF32C is the stock plug. Gap is recommended at 0.050" to 0.054" if I recall. He could also run and Autolite 104 or 764 plug in those heads.
If he's going serious with the setup (compression, aftermarket ignition, etc.) then he might try one heat range colder in the AWSF22C or Autolite 103 and try gapping the plugs down to about 0.045". I wouldn't do that though at the stock compression.
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Jeff Chambers
Trophy Stock #3
11.462 Seconds / 117.43 MPH
Chambers Racing Team