605's are plugs. Just one heat range colder than stock.
If your just going to be running a stock engine with just headers, keep your OEM plugs.
Cam (FFDP) explained it right, its kinda weird sounding but when modding, you want to run the Hottest - coldest plug possible.
You want the hottest plug for your cars setup.
If your stock stick with the OEM plugs such as Autolite 606 Coppers.
Once you start dropping pulley sizes you want to drop to a "colder" plug but is still the hottest you can run for your cars set up. A basic 3.5 setup will require plugs such as Autolite 605's, one step colder than stock. A more "built" engine will require AL104's and even more "built" engines will need AL103's.
