D port manifolds
 

Are the D port ram air manifolds one and the same for GTO and Firebird?

Depends what Firebird you're asking about, and what year A body you're asking about.

1967 A body is one year only the difference is the outlet is further back to clear the crossmember under the engine. In 68 the engine placement and crossmember was changed and the outlet was moved towards the front of the chassis. That also fits second gen Firebirds.

First gen Firebirds have a long branch setup similar to the B body long branch manifolds, I can't say for sure if the 68-72 A body manifolds will fit the first gen chassis, I never tried to make that swap. Because the steering box is in the rear of the front crossmember (rear steer) I would bet against the A body manifolds fitting in a first gen Firebird, but since I've never tried it physically I can't say for sure.

I have used 70 A body RA manifolds in a 71 firebird, so I'm certain that swap will work. I' also tried to use the same manifolds in a 67 A body chassis and it required major cutting of the crossmember to get them shoehorned in there.

Hope maybe some of my firsthand experience helps answer your question.