EGR Delete 1971 340

I made a block off plate for an EGR opening on a 340 intake, it leaked Exhaust all over the intake manifold.
This is Hi-Rise Intake from a 71-72 Duster. I cannot find a part number for the Gasket, so I made a gasket from a sheet of Cometic C15180, specs say it is good to 1200 Degrees F.
I think it leaked from both sides of the Gasket.
Should there be a sealant on that surface?
Is 1/8" Thick enough for the Block Off plate?
dkamp