That setup, with the HEI upgrade including a distributor carefully curved (by DusterIdiot over on slantsix.org ) for your exact needs, and a thoughtfully-selected and -tuned carb, should do very well for the service you describe. There will not be a big difference in fuel consumption at low idle between a 170 and a 225. Those "warm-ups" and short commutes will result in poor gas mileage and quickly-dirtied engine oil no matter what.