No one installing headers on an A-body should expect them to fit without modifications. I don’t think that’s a secret.
Doug’s and TTI’s are by far the best options, and I have seen both of them fit some cars without any modifications. But I have also seen both of them require modifications on cars that, at least on the surface, were set up identically to cars that they fit without mods.
There are significant body tolerance variations on these cars. And there are production tolerances on Doug’s and TTI’s too. And there are production tolerances on engine mounts, and everything else that has an effect on how well those headers fit. That’s just how it is, you can’t make anything without some level of production variation. The more modifications you make, the more interference issues you’re likely to have.
