If I had a 273 Dart , i would keep the 273 , how often do you see 273 Darts ? people all stick a 360 or 340 in them . 273 is plenty good for a street cruiser , good heads a cam and you're set . Put in somenew torsion bars and good bushings , maybe new rear springs upgrade the brakes and have fun . That car will be fun to drive , and different