Carburetors are not as efficient as fuel injection, but they used to be all there was and we didn’t think twice about it.
I never had much luck with Holley’s although they are pretty easy to work on, just swap some parts around. When I became more interested in some sort of economy, I went back to the factory (Rochester) Quadrajet. It came in two sizes with the smaller CFM being the most common. They have a relatively small primary venturi and huge secondary. Cruising in town and steady highway driving lets you stay on the primary’s, but when you are ready step it up a little, those huge secondary’s can pin you in the seat.
That being said, none of the carburetor’s are perfect out of the box, unless you just want the engine to run and not much more. Fiddling with and improving them takes some education and dedication. There are (were) good books on the subject.
If you were pretty happy with the Demon on your 307, put it on the 350 and see how it works. The engineers you mention, were they affiliated with Holley by any chance? I don’t really know what engineers you are referring to, but take their advice or comments with a grain of salt, learn and understand how a carb really controls air/fuel and make your own experiences.
A carburetor uses vacuum created from the intake stroke of each piston to control the amount of air/fuel drawn into the cylinders. A lumpy cam doesn’t allow for good vacuum (at low speeds and idle) so the carb is most always setup rich (poor mileage). A dual-plane stock manifold will usually give you the best low to upper mid-range performance.
Just a comment: aftermarket fuel injection is a serious replacement for the carburetor when you’re ready. Much finer control of the fuel/air, more precise metering, simpler mechanically, not as simple electrically/digitally. Costs a lot more (although I think the price of a new carburetor is astronomical).