AC Delco Acniter R44 is the go to plug for Z/28 as far as I know. Many people run the hotter R45 and some have tried the colder R43 plug but they're usually not well suited to the street and tend to foul.
I got a set of R43 off eBay a couple years ago and last year got a couple sets of R44 from another member here on the forums.
