I think all of the new master cylinders will be cast iron. 7/8" strait bore master cylinders where used on manual brake equipped g-body cars from 1978-1981. 24mm strait bore cast iron and aluminum master cylinders where used on 1978-1981 vacuum assisted g-body cars. New master cylinders will be cast iron. Aluminum, 24mm strait bore master cylinders where used on some 1980-1981 g-body cars. My 1980 El Camino came with a 24mm bore aliminum master cylinder. From 1982-1988, g-body cars where equiped with step bore (24mm bore and 1.25" bore), aluminum master cylinders that were used with low drag calipers.
Strait bore and step bore master cylinder used different vacuum boosters. If you go with a strait bore master cylinder (recommended), make sure you get the matching vacuum booster.
