Had a 1974 280C, same basic engine/carb setup but without cats. Mine was running excellent and still only got about 14MPG on the highway. The US version of these engines are really bad on gas mileage and only have 120hp versus 157hp on the euro which gets about 20MPG on the highway. Regarding the smog equipment, 1976 vehicles are still required to pass smog, only 1973 and older are exempt. Unless you can get a knowlegeable tech to work on the Solex, you are better off converting to Weber or Holley.
I believe for 1976 the cat is down the exhaust pipe not on the manifold. Hope this helps.