It seems to me, no, the center-dump manifolds won't fit the G-body frame. I know the rear-swept "truck" ram's horns DO fit, as they're illegal in a certain circle track class I build for. They are a bit smaller at 2". Call a wrecking yard with an interchange manual and they MAY be able to tell you. A local Chevy "guru" would be the best person to ask.
What is a TH 400 R? Are you refering to the Turbo-Hydramatic (TH) 400? That's a 3-speed heavy duty auto trans. Or maybe TH700-R4? That's a 4-speed (OD) auto, light-to-medium duty.
Knowing what intake it is would be helpful. There are dozens of aluminum intakes for SBC.
Next? Internal engine parts? "Bolt ons"? Be more specific.
Jim