Small block mileage

My 67 Dart 273-2 barrel, 904 and a 2.71 8 1/4 rear, 225/70-15 tires got 21 on my last 1000 mile road trip. The 273 has never been apart, never had the valves adjusted with 73,000 miles. Oh! this is with the AC on also. It is gutless, can't even spin the tires-but Oh well.