Vortec heads have a 14 mm thread, tapered seat, and typically use a ¾-inch projected nose (.750) reach, small (5/8") spark plug socket. Some suggestions:

AC/Delco are the factory stock plugs and I have had good success with them.
R42LTS - Stock and a good place to start.
R43LTS (colder)
R44LTS (coldest)

NGK
TR5 (stock equivalent )
TR55 (colder)
TR6 (coldest)

Autolite
103 (stock equivalent)
AR103 (Racing)
AR104 (Racing)

AP605 (Platinum)
APP605 (Double Platinum – kinda pricey)
XP605 (Xtreme Perf)

Bosch (0.69" reach)
HR9DC
HR8DPX
4305
4458
4205

Here’s a good (really complete) xref:
http://www.angelfire.com/extreme4/sk..._REFERENCE.htm