Opinions

I do not think this is a carb problem. It is certainly strange & interesting...

On a DP intake, each plane [ & therefore one side of the carb ] feeds two cyl on the left bank, two on the right bank. Same with the other plane. The fact that shorting out the plugs on the left side almost stalls the engine, but the other side doesn't makes me think the problem is left side [ odd, 1,3,5, 7 ] related.
First thing that comes to mind is int gasket leaking on left side.