big blocks drink alot of fuel, not really any way around that, a good .21 is probably your best bet for fuel economy, and if a decent enough one will not have to sacrifice too much in the power department. (think .21s are the main engine class for racing)
higher quality nitro engines seem to not need tuning quite as much, though when something changes either temperatures, elevations, humidity, etc. you are going to need to adjust the tune. running higher (25%-30%) nitro content also makes them less finicky on tune.
i know the Novarossi Rex Legend 28-8 is a good engine, heard alot of praise about it, along with about any Novarossi, dynamite makes decent engines, but definitely not quite the quality of the aforementioned, they do make a .26 though which is right there in between.