Skip the rubber and just lay a bead of HTV silicone on the block after you have it clean. Clean the bottom of the intake also, using a good solvent. Let the silicone set up 10 minutes or so, then bolt down the intake. This has worked for me on an engine that was my dally driver for over 8 years. I just tore that engine down for a rebuild and that area never leaked at either end of the
manifold. I'll be putting it back together the same way.