105? 604?

Random numbers...I'm lost there...

The Autolite's come in 606, 605, 104, 103...from hottest to coldest in that order IIRC...
The NGK's are either the TR55, or TR6...

IMO I'd get the Autolite 605's, or if you want a colder plug than spring for the 104's...but since you just have a GT I wouldn't worry about it...605's are a great plug if you have bolt-ons I'd guess.