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They have been making headers for these applications for so long I am surprised you can still buy a set with fitment issues for sbc applications.
I have Headman Headers on my build. They fit perfect. I had to move the gas lines and a brake line on the passenger side. I was able to put the passenger side on from above and the drivers side from below after removing 2 spark plugs. I have a double hump on my trans cross member (one hump on each side). My headers lined up with the humps pointing under the cross member.