My car first smell like gas so I changed the gas tank to filler hose, theold one looked ok on the outside but jesus was the rubber warn and cracked on the inside, Gas smell went away. Re got the gas smell this time was due to leaking accelterater pump leaking gas on to the inlet, replaced that nogas smell.

A cat may make things worse smell wise until up to temp as it will produce H2S (bad egg smell). You may also posion the cat very quickly as the emission of these engine are probably way higher than an engine that has been designed for a cat, so that would shorten it's life. Check also the hood to fire wall seal is good. A trick a friend uses for new cars (he get a new car ever year or so) is the old baking powder one which absorbs smells. He also claimes dried used coffee grounds also work for this but ihate coffee so I would neveruse that trick!!