Use a good file and "draw file" the mating surfaces of the manifold and water neck so they are dead flat, use a regular good quality off the shelf gasket, and some heat and water resistant non-hardening gasket sealer (I won't debate the differences between sealer and RTV here), new bolts of the proper length and use the same sealer on the bolt threads; don't over tighten the bolts, and use a torque wrench to make sure they are tightened equally. I've heard it said the God (or, the devil) is in the details; this is one place details definitely pay off, or will bite you in the backside. And, as said otherwise, those fancy chrome "pot metal" water necks are pure junk. Best to use a neck of the same material as your manifold, and if a shiny pretty is called for, get an expensive "manufactured" one, made up from plate and tube that has been fit and welded and surface milled; stainless is a good thing for this.