It depends on the year. Initially Z-28s had 302s for SCCA T/A homologation. In 1970 they got 350s. Later Z-28s could have low performance 305s or 350s. None of the 1980s Z-28s performed well, even with the TPI 350. But, of-course, in 1993 they got the LT1 350 and performance was back.
The R/S package was not an engine option package.