Here are correct engine codes for 1968 Firebird with 400 engine:
WZ- 400 MT YT- 400AT WQ- 400HO MT YW- 400HO AT WI- RA I MT (D port head) XN- RA i AT (D port head) WU- RA II MT (Round port head) XT- RA II AT (Round port head)
Engine codes are stamped on passenger side of block around timing cover. Block casting # is on back side of block passenger side along transmission, hope this helps.