chevy engines

[Roy Alan Oxlade]

Hi

I know this aint strictly efi but you will know the answers, as it
concerns an American vehicle and I am from the UK..

I have a 1976 Pontiac Firebird with a 350 CI engine, is this engine GM
or Chevy?

How can  you tell the difference between the two engines, I was told
Pontiac used Chevy engines in some Firebirds but when or in what models
I dont know.

Is there also any obvious differences between the 350, 400 & 450! ?.