I'll give you the # off the one I installed...Deutsch PCV118. That's equivalent to NAPA CRB 29213 per NAPA's online catalog.

CRB 29281 would be correct for a '68 Firebird with its original engine.

AC-Delco lists the Deutsch PCV118 as being equivalent to AC-Delco CV679C which is called out for many years, makes and models through 1984, as well as CV768C which was a Buick-only application.

Unless the engine is grossly out of spec or the valve is stuck one way or the other, I don't think it's a real issue on most large-displacement engines. The valve I removed went into the trash, so I don't know whether it was stuck open or leaking, but replacing it did solve the problem I was having.