choke block off plate.

I think you're talking about the EGR blocking plate. The choke sits up in a well on the top of the manifold, and usually doesn't leak since it is on the outside of the manifold.

If it's the EGR hole (on the side of the exhaust manifold, below the carb), you need an EGR blocking plate. The one I have is really thick (like 3/4") so it won't warp and blow out the gasket. There is one now on eBay for $22.

If the blocking plate you are using is thin it will warp under the heat/torque/pressure and blow out the gasket. ALso, I don't think aluminum is a good choice for anything in an exhaust system, especially a pressure boundary. It expands/contracts too much, cracks under heat, and corrodes when exposed to exhaust. The eBay one is stainless steel.

Use a new gasket, with good prep on the exhaust manfold seating surface, and torque it down with no sealant. Should work for you.