I once had a (4-yr.old at the time) 'one-owner' low-miles '78 Dodge shorty van. It had 3spd. (floor-shift) and the Super Six. It was the first and only factory 2Bbl. six I would ever own. It ran leaner than it should have so I installed a BBD from '74 318. I noticed right away when comparing both carbs that the 318 unit had larger venturiis. So, in this case, especially with 'bigger' cam profile, the 318 carb could be the way to go, however, it would need to be 'dialed-in'.
Agree also that vacuum leak(s) could be an issue and suggest to use a vacuum gauge while doing adjustments.
