Huge Pontiac question

I have a 68 Pontiac 350bb and it makes my sons 76 Pontiac 400 look like a 6cyl and I read from both sides that they never made a big block but then I read that is all they made for 350 and they based the 400 off from that block so my questions are. In 1968 Pontiac Firebird come with a 350bb even after typing in codes? Did the 1976 Pontiac Trans Am come with a 400 and why is it so much different than the Pontiac 350?