The Fenton company, way back made cast iron headers for just about everything at one time. Today a company, Patrick's in Casa Grande AZ repops the Ford Flathead V8 and Chevy inline 216/235/261 headers. I'd check with Langdon's Stovebolt, ad runs on the top of this home page. I don't believe Patricks has a web site, but you could run a search. There are other companies that make or 'split' duals for Chevy sixes also.
Best of luck
