A bit of advice on manifold installation. Chevy had problems with leaky manifolds on their inline six engines due to the design. Several years back I started installing them with Ultra-copper RTV. I place a thin bead (about 1/4" thick) between the gasket and the manifold, then bolt the manifold on and tighten the bolts. Inspect for gaps, and redo with more sealant if there are any. Don't start the engine for 24 hours, allowing the RTV time to cure. At that time you can trim away the excess with a sharp knife or razor blade if desired.
I haven't had a leaky manifold since.
