building a long distance touring car

I'm building a 1970 Dart specifically to use for long distance touring, like taking 101 down the coast to Rt66 across to Chicago then hitting the Nats and Chryslers at Carlisle and then doing it in reverse the next year. I'm trying to figure out the best small block to use in front of a 4 speed overdrive automatic. I'd like to get some decent gas mileage (20-24?) but don't want it to be a dog either (the low 1st gear in the trans will help). So throw out your thoughts, please! I've considered everything from a crate 408 to a 273 power pack style motor.