Got Shell 93 Octane and now barely idles 1973 Dart 340

My vote is for an air/vacuum leak

I was thinking that as well. The previous owner doubled up 2 gaskets on the stock manifold and then a adapter plate and then a thin gasket. There is even evidence of red RTV. My assumption the reasoning was because of replacing the stock Carter Thermoquad which is a spreadbore? Please see the pics. Is there a way around this without changing the intake?

Thanks,
Eric

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