There must be a trick to sealing that connection where the intake and exhaust manifold sit together right below the carburetor. It seems that every slanty that I have worked on leaks from there.
On mine, I cleaned it SPOTLESS, then got a sanding block and leveled both the intake and exhaust manifolds out. Then I sprayed the thin steel gasket with a THICK coat of Copper Kote and let it sit overnight and assembled. So far, no leaks at all.