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Edit: (again) You have the wrong gasket, or a gasket that will not accomodate the GT pattern corrrectly.

I can't say what is the correct gasket for your needs, but looking at your pics, I see the gasket has poor matching of the ports as it get closer to the rear of the manifold towards the outlet. The bolt hole used to align gasket is the incorrect bolt hole. GT manifolds only use the bolt holes on the sides of the port, not the ones at 12 o'clock and 6 o'clock. Move the bolt to the correct hole and see if alignment is better. It may be the pic, but it looks like the gasket will "square up" with the ports much better. Also the pic does not show the bottom of the ports. Is the gasket protruding into the exhaust flow on the bottom of the port? On my 390, I have the heads drilled for the CJ 16 bolt pattern. I have difficulty finding the correct bolt pattern and matching gasket that fits my ports. I have had to do a little "trimming" to remove some gasket material from the bottom of gasket so it did not intrude into the ports. But that is because of my LATE MODEL FE heads (D2EAA). This is just a guess on my part. Maybe more pics?

I am going to be going through this issue again soon as I am rebuilding the 390 currently. I ordered the gasket set through the machine shop, and have not even opened it up to see what gaskets I have been supplied. I may be purchasing new gaskets despite having some in my rebuild kit....dang...

NOTE: I also forgot to mention that the 14 bolt GT pattern is different than the 16 bolt CJ pattern in 2 different ways. The first difference is of course is the 16 bolt has 2 extra bolt holes. But the second difference is the 2 outside top bolt holes are located lower than the two inside top bolt holes.

Your bolt is in the wrong location for GT Manifolds. It is in the CJ pattern with the outer hole located higher than the GT
 

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Question: do you also need the spacer surfaces trued as well, and then assembled w/o gaskets?
 
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