273ci--a keeper in my 64 Dart GT 4-speed?

Dear all:

I have acquired a 64 Dart (GT) factory 4 speed / 273 ci motor. It is sufficiently rare that my instinct is to not up-size the motor to, e. g., a 318 or 340, but rather keep it as the factory outfitted it. It would probably used as a daily driver, so fuel efficiency is an issue too. Is the 273 (currently not running) inherently able to be rebuilt to upgrade power/efficiency enough to make it worth while and would replacing it with a 318 or 340 be a big step up in terms of power to weight factors and in terms of potential for modest performance upgrades. It is a project car, and one with a factory configuration that is not plentiful. I already have two beautiful slant-six A bodies which I am planning to convert to dual carbs plus exhaust improvements but I am new to the v-8 world. Any wisdom to share? many thanks. JACOBI