How did people run .............back in the day ? Because that's all there was at the time . There wasn't any choice . It was normal to us . We were unaware that there would be better in the future . It was simply a fact of life that you could have either a deep first gear, or modest cruise rpm , but not both . 12-15mpg is within the ball park for non-OD 3.73, V-8 .
If you went to the effort to swap a V-8 , and imply a heavy foot ,you want a certain level of power . If high milage was your primary concern , you'd run something like a 3.0 .
I'm an old school guy , with more than occasional Luddite tendencies . ( insert tales of Full Size Bronco w/ 351, C-6, 3.55 , and Chev C-10 , 350 , SM465, and Carb ! ) . But even * I * , finally realized that OD is needed for anything with umph , and late 20th century expectations of cruise rpms . ( FSB had 3000rpm cruise , my last several DD Explorers had 1900-2000 .)
Back on topic , back in the day , " compromise " ratios included 3.42, 3.55, and 3.73 . Would 3.55 to increase 1mpg satisfy you / seem like meaningful return on efforts ? Would 2mpg increase w/ 3.42 be acceptable *to you* trade off for the reduced power ?