You do not need to change either of the manifolds to put duals on a '61 or '62 full size car. The parts book lists one LH manifold number and one RH manifold number for '61/'62 applications - single exhaust, dual exhaust or Starfire, doesn't matter. '63s & '64s are different.
See post #6 here:
http://classicoldsmobile.com/forums/...n-exhaust.html
Paul