23° refers to the angle of the intake and exhaust valves in the cylinder head with respect to the deck surface, or the center axis of the cylinder. The valve pockets in the crown of a piston are relief cut at a specific angle to make the appropriate room for the valves while they're open so they won't collide with the piston. The vast majority of Chevy heads (e.g., factory heads) are 23° heads. 23° heads match 23° pistons. Another popular option is an 18° valve angle, which is most commonly used in racing applications.
Standard SB aluminum pistons will work as long as you don't go wild on the valve lift/duration.
You might consider aluminum full roller rockers as a worthy option to the stamped steel rockers.