I took the picture of the gasket on the cylinder head. I am currently running those exact MP R/T heads with freeflows on my Duster and I had to cut "adapters" to get them to seal. So yes, I have personally tried and compared the stock gasket to an R/T head.
That I can agree with. Everything bolts up and lines up, for sure.
The difference I am concerned about is the size/shape of the port and the fact that they are not the same. And the material below the port on my head was lacking enough for me to not just bolt on the HP manifolds without doing something extra.
BTW: the last link you show is for the early A's which use different headers than 67 and up. They may be a problem because headers for them are different. Mine is a 67 and up style
Also in the pic in the link you provided the person is comparing them to stock flange gaskets, not header gaskets which are rarely the same. With headers I always use a good header gasket, not stock flange gaskets like that person used
Just to be clear, "that person" is me and I wasn't working on putting headers on the car at the time. And I've never heard that a header gasket had a different port opening than a stock one did.
The one thing I don't remember doing is laying that gasket on the manifold. I know for certain that there would be little or no overlap between the gasket and the cylinder head equaling an exhaust leak, but I can't say that the port on the freeflows wouldn't be smaller. If the bottom of the port on the freeflow was higher, then the right gasket might have had a chance of sealing.
