When I had my 69 Dart 225 w/ 904 auto, I religiously recorded my mileage. I also verified the odometer was correct, against highway mileage markers. I got a consistent 22 mpg hwy at 60 mph. Speed limits were lower then.
This is more of a problem with certain aftermarket manifold/carb setups than with the stock setup, which actually does a pretty good job of providing a rather even mixture to all six cylinders. Certainly better than any of the other-brand inline Sixes with their rake or log style intake manifolds!
TBI is a lot easier than port injection to retrofit, and would definitely be superior to carburetion as long as the hardware and firmware are good. Holley ProJection is garbage, as is the Holley hardware and Chrysler firmware used on the '88-'91 TBI Dodge trucks and vans (3.9, 5.2, 5.9). In the back of my mind I have thoughts of experimenting with an Offy twin 1bbl intake manifold with dual TBI throttle bodies from '87-up 2.2/2.5 engines.