Check to be certain all of your surfaces are FLAT and CLEAN. Inspect the block, adapter and bolt for other defects such as gouges to the sealing surface, cracks and possible bolt stretch. Unless you have history from when it was brand new, you wouldn't believe what people can damage with a wrench or socket

Here is our Superformance Products kit #340-K5 made from the best materials available, however it is no magic elixir for a component that has issues.
I disagree with using any sealers in this area. These are machined mating surfaces so leave it in your tool box