in the old days, the gasket kits gave you a strip of this putty, which you divided into 1/4's and placed in those 4 corners of the manifold. 3M strip caulk would work, but most people, now, just use a dollop of RTV. Did you use the two cork end gaskets, or did you just use RTV? Sometimes the end gaskets can slide in, or pooch out of position between the surfaces when clamping.