Just bought the same thing from Holley, it came with the thin metal gasket you mentioned and another smaller cork-like gasket???
Any idea what that's for?

I had a slight leak from my rear two barrel at that point, couldn't really tell exactly which fitting was leaking it was so little.

I'm waiting on the 4 fuel lines in the kit they sell and replacing the whole pile of 50 year-old connections.

The rear carb. had a nylon gasket instead of the metal one so I suspect that may be the culprit.

We'll see....

P.S. Holley's part # for the 5/16" is 26-27
P.P.S. Just spoke w/Holley about the smaller gasket, it's for the filter that came originally in there. Forgot about that!!
Joe