Summit sells Pypes down pipes that go from the manifolds to the crossmember.
They also sell header (crossmember) back dual exhaust kits that supposedly fit mid 70's GP's. They also have fabricated dual hump crossmembers which you will need.
you would probably be better off buying a factory dual hump from a forum member as the fab'd ones are expensive. Also check at Jegs. They often have the same products as Summit.
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