Sounds like your building something special. I had Cam do my engines, the last was a 421 and i had the same concerns. We went with 28s and a desert tank for mileage, its better on gas but i cant tell you by how much because its the only carbs ive ever used on it and its been riden sparingly. I just ordered 33s for it due to its hard to tune between pipes with these carbs.
I am currently building another 421 serval and i am not convinced i should. I kind of toying with the idea of a 347 or 358 cub or a 392 serval or driveline setup. A guy out here has a 392 DL set up by the guy at DL and its real impressive. The guys at Driveline would do my next build over anyone else if i was using their cylinders, they have done some experimenting and i thought the results were excellent.