It looks like it won't fit but it does. Take out all clecos in the area and spread the parts a little. Just try to get one end of the shim in and slide it up to the forward end and then push the rest of it down into the gap. The hardest part is getting the holes lined up with the other two parts. The top and back edges of the shim should be flush with adjacent parts.
Wings, empennage and fuselage completed
Finish kit in progress
IO-390 Thunderbolt ordered