Use the OEM v.c. gasket if you can, TRUST ME< IT'S BETTER. No need to hold it in with anything, it'll stay in if you lightly tap it into the valve cover. If you have no codes for o2 sensors or catalyst efficiency, LEAVE THEM ALONE. If you put an aftermarket cat in that car, you'll soon have a check engine light because of it. Aftermarket cats are not up to par with the oem part. Change your fluids, plugs, (use NGK or DENSO), PCV valve, clean the mass air flow sensor with brake cleaner (or other solvent that completely dries and leaves no residue) and don't overthink that car. It's not an old caprice that needs all that stuff. You might look around and see if your car is involved on the A0J ECU replacement.