OK, that's a bit different. Any GMC exhaust manifold should mate up OK to your 270 head...but it may not dump in the same place. There are some good resources for used GMC parts at Inliners try Larrowe & Sons or Langdon's Stovebolt. Even if you don't want duals, Kansas Kustoms may be able to repair your old manifold. And certainly don't forget OldCarKook