Carb for a 65 273

efficiency?

That is helped by high flow air cleaners
Better intakes
proper carb size and state of tune for mileage
(AKA slightly lean)
Well preped heads (Not ported, just well done)
Headers help and a great ignition. I have found that a MSD or equal like type ignition is an excellent mileage getter in our old rides.

Foot outta the rear two barrels and a high gear ratio help.