My wifes 77 Firebird with the stock 400 had a flapper.Its a seperate piece that goes between manifold and headpipe. The manifold is flat,the flapper valve has the bell for the exhaust pipe.It is the 3-bolt type.It is in the service manual. Its called E.F.E (early fuel evaporative) system.
The 2-bolt manifolds stopped using the flapper valve.
Craig
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