My Firebird parts catalogue indicates a 350 HO with a 3.55 rear gear had the standard ratio 4 speed. With a 3.90 rear, the 350HO had the close ratio 4 speed. All manual cars that received the 3.90 or 4.33 gears sets had the close ratio 4 speed according to my book.

Code for lock diff:3.55 is ZH, 3.90 is ZK, 4.33 is ZM. Y means Std diff, Z means lock diff, last letter is gear ratio.