Spread bore carbs
The spread bore carbs were developed to help drivability, fuel economy, and emissions. Carter made the Thermo Quad and GM had the Quadra Jet. Very small primaries and huge secondaries. If either is setup properly the car will pull smoother, get a bit better mpg, and emissions should be better. The old square bore carbs the primary bores were slightly smaller then the secondaries and MPG and emissions were not a concern when they were designed in the fifties and sixties.
I run a ThermoQuad on my Dart and it is the smoothest, best pulling carb I ever have had. I tried a Holley, AFB, AVS, and I tuned all of them the best I could and the TQ is the best of them for me. The spread bore was the most advanced design of carb Americans came up with but most of their runs they were smogged out to death, tied to early horrible emission based computer control and severely locked down as far as adjustability.
Early ones can be tuned very well if you can find a strip kit for it. I have them for the AFB, AVS, and TQ. You have to use a intake designed to use them, most aftermarket later ones can use square bore or spread bore carbs interchangeably. Otherwise you use what the car came with but they did make adapters that sorta worked.
Test drove a ram charger suv that had a 2bbl to 4bbl adapter on it and a small holley. Had to be horribly in efficient and choked down. It did seem to run ok though.