Some 1975 production data
FIREBIRD: 22,293
ESPRIT: 20,826
FORMULA: 13,670
Trans Am: 27,274
TOTAL: 84,063
Only 857 cars had the 455, and it was down to 7.9:1 compression and only 200 HP by this time, only a shadow of its past brutality
20,286 cars had the 400
Those numbers don't total 27,274, so I don't know if they are wrong or what the remaining 1975 cars would have had for engines, maybe the Olds?