They are. In 99/2000 I built a 79 Ford F150 with a 400 and several hot rod parts( Edelbrock intake/Holley 570 Vac secondary/ MSD Digital 6 ignition) and all emissions gear stripped. We took it to a station to have the smog check done for the hell of it.
The guy said it was cleaner than many new cars of that era.
Edit - Holley 670
I had a 86 CJ7. Left the stock motor in. Plugged every vaccuum tube, removed all emmision controls and yanked the cat. Passed with flying colors.
I bet. A well taken care of, well tuned engine can run very clean.