What about having a muffler shop bend a section of exhaust tubing and then connnect it with very short lengths of hose and clamps?
My brother in law has a pair of trucks from this era with step sides that I’ve done work on but simply can’t remember exactly what the pipe looks like. Whether the above suggestion would work or not I have no idea.
(No, he’s attached to them and won’t sell them.) 
You might consider checking with CTC Auto Ranch in Denton, TX. They have acres of old stuff out there and may have a good used part. Hope that helps.