1971 Duster exhaust smell in cabin after new muffler and tail pipe

Also double check all the firewall plugs while you're at it too.
I had that problem as well. I mean it wasn't bad for me, but occasionally I had a slight exhaust smell in my cabin every time I cruised at low speeds or when I sat in traffic for more than a few minutes. It wasn't strong but it was enough to get a hint of it.
Come to find out I had a firewall plug that was dry-rotted and was about to fall out of it's place. I just RTV'ed it temporarily until I got a replacement for it at Classic Industries.