Catalytic Converter on a 65 Slant 6?

Yes, I have reservations of putting a CAT on it too.

I have adjusted between very lean, too rich, everywhere in between. Spark plugs are clean, dry with no deposits.

Oil leaks are under control. It is a smell from the exhaust pipe not a dripping leak.

Does synthetic make a difference on this? I had not thought to try that. Right now, I am running 30w with 1 quart of Lucas.