The company posted above is the one that makes and sells the RA/HO manifolds. What year Firebird are you talking about? RAR covers pretty much all options. The 1967-69 'long branch' manifolds will fit all 1967, 68, 69 Firebirds with any Pontiac V-8 engine, even the 301. They will also fit the 1961-70 'B' body 'big cars' like the Bonneville. This is because these models have the steering box behind the axle, close to the firewall. The second gen Firebirds and 1968-70 GTO will use the ones that dump towards the rear. These 2nd gen models have the steering box ahead of the axle. These will also fit 1978-88 'G' body cars like Grand Prix and LeMans. Seems to me for $600 you can buy brand new manifolds for less. I have seen original manifolds with severe cracks in the internal dividers. The re-pops are made of much better metal and come with a warranty. See this site for more info: Ram Air Restoration