All depends how driven, how loaded & how tuned. Our '69 Fairlane 500 wagon (3600Lbs), loaded with all options, 351 4bbl & dual 2 1/2" exhausts, plus a 27-gallon auxiliary fuel tank would always get superior miles per gallon to our '69 Torino Squire (3400Lbs) with fewer options and a 302 2bbl. Both cars had the same auto trans ratios, as well as 3.00 rear end gears. So how could the 351 beat the 302 on fuel mileage?
Driven reasonably, in a mix of traffic conditions, 1) the 4bbl can usually run on its primaries, smaller than the 2 venturis of the 302's 2bbl , 2) the 351's longer stroke gives it more low speed pulling power (can move the car into the higher gears more quickly), and of course 3) the 351's low restriction dual exhausts didn't hurt either. Obviously, if the power of both engines was being used to the maximum, the 351-4bbl could drink more fuel than the 302-2bbl, at least until the driver(s) were caught & handcuffed!
J.R.
SoCal