SendCutSend can cut stuff for you. I have a pattern I gave someone here recently for this exact reason.
Can't promise it is a perfect fix, I have the same problem (Magnum R/T heads with 340 manifolds) and my adapter plates seem to be ok. But I have leaks I haven't identified so it could be them.
I think I used 14 or 16ga plate for mine, I didn't want to push the manifolds out too far.
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