I am not an expert, but have done a little work on manifolds, what I would do is pull off the intake clean it good and have somebody with an aluminum spool gun put a dam about 1/8 high in front of runners 6 and 7 then dimple the floor of the intake and runners with a course carbide bit to help break up puddles of fuel. I don't think any kind of spacer will fix your issue. If you have access to a flow bench try setting the manifold up and using a wand with a string attached, may help decide exactly how to place the dams, and see how much turbulence is in the plenum. I like a lot of turbulence in the plenum and then the flow should straighten in the runners, if that makes sense.