Just an FYI that may or may not help your situation. I put a 4.3L in my CJ3A. I originally went w/ the stock exhaust manifolds which were a PITA. I wasn't happy and wanted outside the framerail headers. I got to looking around at what V6 headers were available. I found that Buick V6 headers for a CJ5 were the closest I was going to get.
The spacing between the header tubes was very close to the spacing on the 4.3L V6 heads (tried to minimize the amount of fabrication and tube bending). I went ahead and got the headers, cut the head flanges off the headers, made new flanges, using a FelPro 4.3L V6 exhaust manifold gasket as a template for the exhaust ports and bolt pattern. The exhaust ports were rectangle shaped on the Buick headers, so I rounded out the header tubes where they were rectangled and welded the new flanges to the header tubes. Worked like a charm!
Granted you can't run out and buy exactly what you want, but it is possible to fabricate them. I bet I only had 5 to 7 hours in the whole project. Now I have the headers I wanted and decreased the exhaust routing complication. I just had the headers ceramic coated this week so they would last.