And a happy Easter to you.
How many ports are sucking? Is it always the same ones?
You say the manifold sits down flat on the china wall.Im gonna assume that you mean parallel to it but not touching it.Pictures of the gaskets showing the leaky areas would be nice.Im also gonna assume the intake was torqued in the correct sequence and to the correct spec.
I can think of 2 things I would be doing next; checking for rocking motion and claying the joints.
1) after cleaning everything up, and without gaskets I would drop the manifold into place. I would then attempt to rock it in various directions. There should not be any rocking. Heres where Im going; I heard that the heads and manifold were surfaced. Well sometimes the machine shop doesnt get it quite right. The angles can be perfect. Slightly more can be shaved off at one end than at the other. Then when you bolt the heads on, the angle can still be perfect, but the head is lower to the block at one end than at the other.Then when you go to bolt on an unmachined manifold, it has to twist to conform to the surfaces. Next, if the manifold is also machined more at one end than at the other it could aggravate the problem more. Remember that the angles could still be perfect and this could still happen. So the rocking test, although it cant prove this, it can very definitely point to it.
2)I would next clay the joints. I would brackleen the 4 corners of the heads right next to and on the outboard edges of the front and rear ports, and where the heads meet the intake fully from top to bottom, without passing through any voids. I would roll out some modelling clay into about 1/8 x 2 strings, and stick them on there. I would flip the intake upside down and apply a thin layer of slippery substance on the surfaces exactly where the clay is going to touch, so the clay wont stick to it. Then, again without gaskets, I would carefully, and gently set that intake into its final position.Once its down dont shift it.No need to shift it to where the bolts go in, as long as the clay is able to be flattened by the next step. The last step, then is to apply a goodly amount of pressure down onto the carb pad, to compress those clay strips.Pushing down on it from the rad area or the fenders wont do it. The pressure needs to come straight down from above. I would put an empty 5gallon pail on there and fill it full of water. At 10 lbs per gallon that should be enough. Ok so carefully get rid of that pail, and remove the intake. The clay should tell the story. To read it, cut about 1/4 inch off the ends with a razor blade, and remove the those blobs. Then you can see on the heads whats going on. Take a sheet of paper, and mark off 4 quadrants,orient it and mark it as to front/rear/sides. Start in one corner and measure with a caliper,the top and bottom thicknesses of the clay. Write down the two measurements;top and bottom. Repeat for every corner, and write it down. Then take a seat an analyze the results.
If everything was perfect the clays would all be the same thicknesses from top to bottom and corner to corner. In your case, I might expect 2 corners 180* from eachother to be very thin, and the remaining corners to be somewhat thicker. I would also expect differences in the measurements from top to bottom. If enough pressure was applied earlier then I would expect,and would hope for, some zeros or near zeros in there. In any case stare at the results for a while, then go look at the clay. In just a while, a picture will develop in your head, and I think you will see the issue.
If there is rocking, or proof of accumulative machining errors, I dont think there is a cheap fix.
You dont mention that the decks were machined. But if they were, that could also compound the error, again with the ends rising or falling.
So good luck.
