When I was in high school back in the 80's, I put a 340 cam and a Quadrajet off a 350 Buick on my otherwise stock 318 Duster. I had no experienced gear heads around to help me tune the spread bore carb. From what I remember, I lost off-the-line torque when swapping from the tiny Carter 2bbl, but hang on when those big secondaries kick in! Love that ROAR! Tiny Carter 2bbl would roast one tire on the open rear, but not so much after the spread bore was added. This was probably a tuning issue that I didn't know how to correct, or possibly too much carb for the 318 at low rpms. Intake was a OEM cast iron unit from a '71 340.