4 hole phenolic is all you need. That, and remembering that you are driving a carbureted engine, not computer controlled EFI.

How thick do you need? It depends on the engine and your environment. Years back, I found a bunch of 1/4" paper gaskets with the 4 holes. What I'd do is start stacking them up until I found the best thickness, then bought the phenolic spacer that most closely matched.