I'm having similar issues but want to replace all the gaskets from the manifold back. I am pretty sure the manifold gasket isn't leaking, but will perform the smoke test this weekend. I want to get all the flange gaskets, from the manifold to the crossover pipe, cat, etc to the muffler. I can't find a good diagram, even on camelback, of the gasket or donut part numbers. I had the rear cat off a month ago to fix a few leaks in the pipe so those gaskets were replaced, but the ones forward of that up to the engine are suspect still.