I have Hedman headers on my 66 LeMans 326 and they fit ok. I did a body off restoration a few years ago and build the engine and mounted it on the frame before the body went on. I switched over from automatic to 4 speed tranny so I had to work around the clutch linkage. It went together but was tight. The issues I having now is that I have to pull the engine due to a spun bearing on the crank-piston rod and have to try to get everything apart. The passenger side was a breeze, but the drivers side is a royal pain. The main thing that is in the way is the header. It's so difficult to remove everything and now I'm considering replacing the headers with stock type of exhaust manifolds just to be able to put the car back together after the engine repairs. . So if anyone has an old set of exhaust manifolds for a 326 or 400 pointiac, I would be interested in buying them.