Big ole Hemi with a big old Blower in a big old 40 = Pure Class
Big ole Hemi with twin turbos in a big old 40 = interesting but not near as much class

Gas milage of 2 4bls depends on setup and how you drive it. It does not have to be bad. If you size the carbs right and get them spot on, it won't be that far off from normal driving with one carb. If there are plans for a set of rubber tracks coming out of your driveway that get thicker by the week, quit being prissy about the milage. Take the "Seriously" route.