All the 4 bbl engines used 2" exhaust except the ram air exhaust used 2 1/4". GM didn't start useing 2.5" until they went to the 82 bird when it was just single exhaust. A 70 G/P with the 455HO would have come with the long branch exhaust manifold & the 2 1/4" exhaust pipes. Otherwise if it were just a plain 455 it would have come with just 2" exhaust.
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